net: stmmac: Add SOCFPGA glue driver

Like the STi 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>
Acked-by: David S. Miller <davem@davemloft.net>
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v3: Remove stray empty line at end of dwmac-socfpga.c.
v2: Use the dwmac-sti as an example for a glue layer and split patch up
to have dts as a separate patch. Also cc dts maintainers since there is
a new binding.