stmmac: start adding pcs and rgmii core irq

This patch starts adding in the main ISR the management of the PCS and
RGMII/SGMII core interrupts. This is to help further development
on this area. Currently the core irq handler only clears the
PCS and S-R_MII interrupts and reports the event in the ethtool stats.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Byungho An <bh74.an@samsung.com>
Cc: Udit Kumar <udit-dlh.kumar@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>