ARM: socfpga: dts: Add tx-fifo-depth and rx-fifo-depth properties

Add tx-fifo-depth and rx-fifo-depth devicetree properties for socfpga
stmmac. These devicetree properties will be used to configure certain
features of the stmmac on the socfpga.

Signed-off-by: Vince Bridgers <vbridger@opensource.altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>