[POWERPC] fsl_soc: add support to gianfar for fixed-link property

fixed-link says: register new "Fixed/emulated PHY", i.e. PHY that
not connected to the real MDIO bus.

Signed-off-by: Vitaly Bordug <vitb@kernel.crashing.org>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>