dt-bindings: dp83867: Convert fifo-depth to common fifo-depth and make optional

Convert the ti,fifo-depth from a TI specific property to the common
tx-fifo-depth property.  Also add support for the rx-fifo-depth.

These are optional properties for this device and if these are not
available then the fifo depths are set to device default values.

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