stmmac: Add properties for transmit and receive fifo sizes

The Synopsys stmmac fifo sizes are configurable, and need to be known
in order to configure certain controller features. This patch adds
tx-fifo-depth and rx-fifo-depth properties to the stmmac document
file.

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