net: Merge asm/fsl_enet.h into fsl_mdio.h

fsl_enet.h defines the mapping of the usual MII management
registers, which are included in the MDIO register block
common to Freescale ethernet controllers. So it shouldn't
depend on the CPU architecture but it should be actually
part of the arch independent fsl_mdio.h.

To remove the arch dependency, merge the content of
asm/fsl_enet.h into fsl_mdio.h.
Some files (like fm_eth.h) were simply including fsl_enet.h
only for phy.h. These were updated to include phy.h instead.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>