fm-eth: Add ability for board code to disable a port

fm-eth: Add ability for board code to disable a port

The SoC configuration may have more ports enabled than a given board
actually can utilize.  Add a routinue that allows the board code to
disable a port that it knows isn't being used.

fm_disable_port() needs to be called before cpu_eth_init().

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>