uboot-airoha: fix ethernet on Gemtek W1700K

uboot-airoha: fix ethernet on Gemtek W1700K

The board enables gdm1 in its DTS, but U-Boot v2026.07 ships
arch/arm/dts/an7581-u-boot.dtsi, which is appended to the end of the
board DTS and declares gdm1 with status = "disabled". The board setting
is overridden and U-Boot ends up without a network device.

Add a board specific an7581-w1700k-ubi-u-boot.dtsi re-enabling gdm1.
U-Boot only pulls in the first matching *-u-boot.dtsi (firstword in
scripts/Makefile.lib), so the board file must include the SoC one
explicitly, otherwise the entire an7581 U-Boot glue is dropped along
with it: uart1 bootph-all, the eth/pcs/snfi/mmc nodes and the ATF
reserved memory.

This is the same fix that was confirmed to restore networking on the
Nokia XG-040G-MD. It is compile tested only and verified by inspecting
the generated DTB - I have no W1700K hardware, so it is unverified on
the actual device and needs testing by someone who has one.

Fixes: baeacca59889 ("uboot-airoha: update to v2026.07")
Signed-off-by: Vitaliy Sochnev <sochnev.v.74@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24410
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>