stmmac: add GMAC4 core support

This is the initial support for GMAC4 that includes
the main callbacks to setup the core module: including
Csum, basic filtering, mac address and interrupt (MMC,
MTL, PMT) No LPI added.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>