stmmac: add the experimental PCI support

This patch adds the PCI support (as EXPERIMENTAL)
this has been also tested on XLINX XC2V3000 FF1152AMT0221
D1215994A VIRTEX FPGA board.
To support the PCI bus the main part has been reworked
and both the platform and the PCI specific parts have
been moved into different files.

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