bgmac: add support for Northstar SoC (BCM4707, BCM53018)

This adds support for the Northstar SoC. This SoC does not have a PMU in
bcma and no register on it should be called. In addition it support 2.5
GBit/s Ethernet to the PHY.

This GMAC core is not fully working there are still problems with the
DMA controller.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>