net: phy: dp83867: Add rx-fifo-depth and tx-fifo-depth

This code changes the TI specific ti,fifo-depth to the common
tx-fifo-depth property.  The tx depth is applicable for both RGMII and
SGMII modes of operation.

rx-fifo-depth was added as well but this is only applicable for SGMII
mode.

So in summary
if RGMII mode write tx fifo depth only
if SGMII mode write both rx and tx fifo depths

If the property is not populated in the device tree then set the value
to the default values.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>