bmips: huawei,hg556a-b: enable external switch

bmips: huawei,hg556a-b: enable external switch

The external switch of the Huawei HG556a is a BCM5325E connected by MDIO.

All the DSA brcm legacy FCS tag and b53 patches have been submitted upstream
and will be backported when accepted.

There are still some sporadic FDB errors, but at least the switch is working
and stable on my device:
bcm53xx fffe4800.ethernet-mii:1e: port 0 failed to add 72:31:59:xx:xx:xx vid 1 to fdb: -28
bcm53xx fffe4800.ethernet-mii:1e: port 0 failed to add 5c:4c:a9:xx:xx:xx vid 0 to fdb: -28
bcm53xx fffe4800.ethernet-mii:1e: port 0 failed to add 5c:4c:a9:xx:xx:xx vid 1 to fdb: -28
bcm53xx fffe4800.ethernet-mii:1e: port 0 failed to delete 72:31:59:xx:xx:xx vid 1 from fdb: -2

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