net/fman: add a fm_enable_port function

This can be useful if one wants to disable an interface in u-boot
because u-boot should not manage it but then later reenable it for FDT
fixing or if the kernel uses this interface.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
[York Sun: fix conflict in fm_eth.h]
Acked-by: York Sun <yorksun@freescale.com>