arm64: dts: Add base DTS file for bcmbca device Asus GT-AX6000

It's a home router with 1 GiB of RAM, 6 Ethernet ports, 2 USB ports.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: William Zhang <william.zhang@broadcom.com>
Link: https://lore.kernel.org/r/20220713200351.28526-2-zajec5@gmail.com
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>