bgmac: reset all cores on Northstar SoC

On the Northstar SoC (BCM4707 and BCM53018) we have to enable all GMAC
cores when we just want to use on. We iterate over all the cores and
activate them.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>