net: stmmac: Add SOCFPGA glue driver

Like the STi and sunxi series SOCs, Altera's SOCFPGA also needs a glue layer
on top of the Synopsys gmac IP.

This patch adds the platform driver for the glue layer which configures the IP
before the generic STMMAC driver takes over.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>