net: stmmac: Add ethtool register dump for XGMAC cores

Add the ethtool interface to dump the register map in XGMAC cores.

Changes from v2:
	- Remove uneeded memset (Jakub)

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>