realtek: add support for Zyxel GS1920-24 v1

The GS1920-24 v1 is the non-PoE, fanless sibling of the already
supported GS1920-24HP v1 (ZyNOS firmware code AAOB vs. AAOC) and
shares its board layout and flash map.

Hardware:
* SoC: Realtek RTL8392M
* Flash: 16 MiB SPI NOR (Macronix MX25L12835F)
* RAM: 128 MiB DDR3
* Ethernet: 24x 10/100/1000BASE-T (3x RTL8218B),
  4x combo RJ45/SFP (RTL8214FC)
* Hardware monitor: Analog Devices ADT7463C
* LEDs: PWR, SYS, ALM, LOCATOR, per-port link/activity
* Buttons: 1x reset (front)
* UART: RS-232 level (not TTL) on header J2, DCE pinout
  (Tx pin 2, Rx pin 3, GND pin 5), 9600 8N1
* Power: 100-240 V AC, 50/60 Hz, 25.9 W max
* No PoE, no fan

MAC address:
* Ethernet: factory MAC from the bootbase partition at offset 0x1fff8
  through the factory_macaddr nvmem cell
* ethernet0 is the label MAC device; 02_network uses this address for
  the LAN/bridge without assigning individual per-port MAC addresses

The hardware monitor at I2C address 0x2e was identified on hardware
as an ADT7463C from company ID 0x41 and VERSTEP 0x6a. The GS1920-24HP
v1 uses an ADT7468, while the v2 uses an lm85-family hardware monitor
at the same address.

All GS1920-24 variants use hardware monitors supported by
kmod-hwmon-lm85. The shared GS1920 device recipe therefore supplies
that package, while the GS1920-24HP-specific recipe additionally
supplies the PSE driver.

Installation:
1. Connect to the console. Optionally switch BootBase to
   115200 baud with ATBA5 (and reconnect at that rate).
2. Load an initramfs image into RAM: ATUP80100000, send the
   image via XMODEM, then ATGO80100000.
3. Copy loader.bin and the sysupgrade image to /tmp on the
   running initramfs system.
4. Flash both:
   mtd write /tmp/loader.bin loader
   mtd write /tmp/*-squashfs-sysupgrade.bin firmware
5. Reboot.

The SFP side of the combo ports is untested (no modules available);
the copper side of all 28 ports works.

Tested on hardware: Flash installation and runtime operation verified.

Signed-off-by: David Evans <git@d4vid.uk>
Link: https://github.com/openwrt/openwrt/pull/24962
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

target/linux/realtek/base-files/etc/board.d/02_network

@@ -97,6 +97,7 @@ realtek_setup_macs()
 	plasmacloud,psx28|\
 	sirivision,sr-st3408f|\
 	ubnt,usw-pro-xg-8-poe|\
+	zyxel,gs1920-24-v1|\
 	zyxel,gs1920-24hp-v2)
 		lan_mac="$(get_mac_label)"
 		;;

target/linux/realtek/dts/rtl8392_zyxel_gs1920-24-v1.dts

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/dts-v1/;
+
+#include "rtl839x_zyxel_gs1920-24-common.dtsi"
+#include "rtl8392_zyxel_gs1920-24-v1-common.dtsi"
+
+/ {
+	compatible = "zyxel,gs1920-24-v1", "realtek,rtl8392-soc";
+	model = "Zyxel GS1920-24v1";
+
+	aliases {
+		label-mac-device = &ethernet0;
+	};
+};
+
+&i2c4 {
+	adt7463: adt7463@2e {
+		compatible = "adi,adt7463";
+		reg = <0x2e>;
+	};
+};

target/linux/realtek/image/rtl839x.mk

@@ -138,6 +138,16 @@ endif
   $(Device/rt-loader-bootbase)
 endef
 
+define Device/zyxel_gs1920-24-v1
+  $(Device/zyxel_gs1920)
+  SOC := rtl8392
+  FLASH_ADDR := 0xb40c0000
+  IMAGE_SIZE := 12144k
+  DEVICE_MODEL := GS1920-24
+  DEVICE_VARIANT := v1
+endef
+TARGET_DEVICES += zyxel_gs1920-24-v1
+
 define Device/zyxel_gs1920-24hp
   $(Device/zyxel_gs1920)
   DEVICE_MODEL := GS1920-24HP