drivers: net: xgene-v2: Add mac configuration

This patch adds functions to configure and control mac.  This
patch also adds helper functions to get/set registers.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>