stmmac: Read tx-fifo-depth and rx-fifo-depth from the devicetree

Read the tx-fifo-depth and rx-fifo-depth from the devicetree. The Synopsys
stmmac controller fifos are configurable per product instance, and the fifo
sizes are needed to configure certain features correctly such as flow control.

Signed-off-by: Vince Bridgers <vbridger@opensource.altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>