dts: socfpga: Add DTS entry for adding the stmmac glue layer for stmmac.

This patch adds the dts bindings documenation for the Altera SOCFPGA glue
layer for the Synopsys STMMAC ethernet driver.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Acked-by: David S. Miller <davem@davemloft.net>
---
v3: Remove stray empty line at end of socfpga_cyclone5_socdk.dts
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.