stmmac: convert dwmac-sti to platform driver

Convert platform glue layer into a proper platform
driver and add it to the build system.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/ethernet/stmicro/stmmac/Kconfig
Simple merge

diff --cc drivers/net/ethernet/stmicro/stmmac/Makefile
Simple merge

diff --cc drivers/net/ethernet/stmicro/stmmac/dwmac-sti.c
Simple merge

diff --cc drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c
Simple merge

diff --cc drivers/net/ethernet/stmicro/stmmac/stmmac_platform.h
Simple merge