net: stmmac: Implement RSS and enable it in XGMAC core

Implement the RSS functionality and add the corresponding callbacks in
XGMAC core.

Changes from v1:
	- Do not use magic constants (Jakub)
	- Use ethtool_rxfh_indir_default() (Jakub)

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