bmips: add support for Sercomm H-500s

bmips: add support for Sercomm H-500s

Sercomm H-500s is a BCM63268 with 128M, internal and external (Quantenna) wifi
and external BCM53134S switch.
This device is already supported in bcm63xx target, so more information can be
found in https://openwrt.org/toh/sercomm/h500-s.

It's a perfect example of a device with internal and external switch
coexistance since most devices only have ports on one of the switches but not
both of them.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>