uboot-mediatek: add TP-Link TL-XDR4288 and TL-XDR608x

uboot-mediatek: add TP-Link TL-XDR4288 and TL-XDR608x

TP-Link TL-XDR608x comes with locked vendor loader. Add U-Boot build
for replacement loader for both TL-XDR6086 and TL-XDR6088. The only
difference at U-Boot level is the different filename requested via
TFTP, matching the corresponding OpenWrt build artifacts for each
device.

The TP-Link TL-XDR4288 has the same hardware as the TP-Link TL-XDR6088
except for the wireless part. Also create a uboot for the TP-Link
TL-XDR4288.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[rebase to uboot 23.04, correct led and button]
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>