uboot-mediatek: wl-wnt100x3: reduce ram speed to 1866 MT/s

The vendor BL2 of the Wavlink WL-WNT100X3 initializes DDR3 at
1866 MT/s:

  NOTICE:  BL2: v2.7(release):292bca1-dirty
  NOTICE:  DDR3 1866MHZ
  NOTICE:  EMI: Using DDR3 settings

while the plain spim-nand-ddr3 BL2 used by the ubootmod variant runs
DDR3 at the default 2133 MT/s, which is above what the vendor
qualified on this board.

Switch the ubootmod BL2 to the existing DDR3 1866 MT/s build, in line
with the vendor firmware.

Tested on WL-WNT100X3: BL2 reports the ddr3-1866 build, the memory
test passes, U-Boot and OpenWrt boot from UBI.

Fixes: d2fabb974c ("mediatek: add support for Wavlink WL-WNT100X3 ubootmod")
Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/25763
Signed-off-by: Robert Marko <robimarko@gmail.com>

package/boot/uboot-mediatek/Makefile

@@ -815,8 +815,8 @@ define U-Boot/mt7981_wavlink_wl-wnt100x3
   UBOOT_IMAGE:=u-boot.fip
   BL2_BOOTDEV:=spim-nand
   BL2_SOC:=mt7981
-  BL2_DDRTYPE:=ddr3
-  DEPENDS:=+trusted-firmware-a-mt7981-spim-nand-ddr3
+  BL2_DDRTYPE:=ddr3-1866
+  DEPENDS:=+trusted-firmware-a-mt7981-spim-nand-ddr3-1866
 endef
 
 define U-Boot/mt7981_xiaomi_mi-router-ax3000t