ramips: add ASUS RT-AC1200GU as alt name

RT-AC57U and RT-AC1200GU are the same models sold in different countries.
The purpose of this commit is to allow users to easily find the
corresponding firmware through the model number on the device label.

More specifications: 14e0e4f ("ramips: add support for ASUS RT-AC57U")

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
[reword commit title/message]
Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ramips/image/mt7621.mk

@@ -162,6 +162,8 @@ define Device/asus_rt-ac57u
   $(Device/dsa-migration)
   DEVICE_VENDOR := ASUS
   DEVICE_MODEL := RT-AC57U
+  DEVICE_ALT0_VENDOR := ASUS
+  DEVICE_ALT0_MODEL := RT-AC1200GU
   IMAGE_SIZE := 16064k
   DEVICE_PACKAGES := kmod-mt7603 kmod-mt76x2 kmod-usb3 \
 	kmod-usb-ledtrig-usbport