dt-bindings: arm: Add Asus GT-AX6000 based on BCM4912

It's a home router, the first BCM4912 SoC based public device.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: William Zhang <william.zhang@broadcom.com>
Link: https://lore.kernel.org/r/20220713200351.28526-1-zajec5@gmail.com


Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>