phy/fixed.c: rework to not duplicate PHY layer functionality

With that patch fixed.c now fully emulates MDIO bus, thus no need
to duplicate PHY layer functionality. That, in turn, drastically
simplifies the code, and drops down line count.

As an additional bonus, now there is no need to register MDIO bus
for each PHY, all emulated PHYs placed on the platform fixed MDIO bus.
There is also no more need to pre-allocate PHYs via .config option,
this is all now handled dynamically.

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