net: stmmac: gmac4+: Add Split Header support

GMAC4+ cores also support the Split Header feature.

Add the support for Split Header feature in the RX path following the
same implementation logic that XGMAC followed.

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