macb: initial support for Cadence GEM

The Cadence GEM is based on the MACB Ethernet controller but has a few
small changes with regards to register and bitfield placement.  This
patch detects the presence of a GEM by reading the module ID register
and setting a flag appropriately.

This handles the new HW address, USRIO and hash register base register
locations in GEM.

v3: - convert to macb_is_gem() inline rather than storing a boolean
      flag
    - handle rx_overrun stats for gem

Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Tested-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>