drivers: net: stmmac: reworking the PCS code.

The 3.xx and 4.xx synopsys gmacs have a very similar
PCS embedded module and they share almost the same registers:
for example:
  AN_Control, AN_Status, AN_Advertisement, AN_Link_Partner_Ability,
  AN_Expansion, TBI_Extended_Status.

Just the RGMII/SMII Control/Status register differs.

So This patch aims to reorganize and enhance the PCS support.
It removes the existent support from the dwmac1000/dwmac4_core.c
moving basic PCS functions inside a new file called: stmmac_pcs.h.

The patch also reviews the available APIs to be better shared among
different hardware and easily enhanced to support new features.

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