gitattributes: automatically convert dts file CRLF line endings to LF

It seems that some developers prefer to use the Windows OS to add new
device support. Add new attributes for device tree files so that the
CRLF line endings can be automatically normalized to LF.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/20978
Signed-off-by: Robert Marko <robimarko@gmail.com>

.gitattributes

@@ -1,2 +1,4 @@
 * -text
 *.patch whitespace=-indent-with-non-tab,-space-before-tab,-tab-in-indent,-trailing-space
+*.dts text eol=lf
+*.dts[io] text eol=lf