generic: ar8216: add support for ar724x/ar933x builtin switch

This builtin switch is a bugless ar8216 with different mib counters
and gigabit cpu port.
Atheros uses the same device ID and it's impossible to distinguish
the standalone one and the builtin one. So we add support to mdio
device probe only.

This switch doesn't have buggy vlan tag so it's not needed to enable
atheros header. This commit changed ar8216_setup_port so that it can
be reused for this switch.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>