ramips: add support for Zyxel NWA90AX access point

ramips: add support for Zyxel NWA90AX access point

The NWA90AX is hardware-wise identical to the NWA50AX which is
already supported.

The NWA90AX magic model code bytes are `77 E1`,
and they are added to the DTS to mark the NWA50AX firmware
as being compatible with the 90 model.

Without the compat-models change, uploading the OpenWrt NWA50AX
firmware with the official Zyxel web interface yields an error:
> errno: -25007
> errmsg: Firmware content error!

As described on the NWA50AX firmware page[1] on the wiki, the
"current image" slot for firmware updates has to be "1".
If it is 0, flashing will fail.

[1] https://openwrt.org/toh/zyxel/nwa50ax

Vendor product page:
https://www.zyxel.com/global/en/products/wireless/ax1800-4-stream-wifi-6-dual-radio-nebulaflex-access-point-nwa90ax

Vendor support page stating that the hardware is identical:
https://support.zyxel.eu/hc/en-us/articles/4416989548178-Access-Point-NWA50-55AXEE-90AX-110AX-210AX-Differences-in-Hardware-and-Features
> NWA90AX: Identical hardware as in NWA50AX, but with added features
> like Captive portal for Guest access and WPA Enterprise for
> AD/Radius (Credential) authentication.

Signed-off-by: Christian Weiske <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20308
Signed-off-by: Hauke Mehrtens <[email protected]>