phylib: rework to prepare for OF registration of PHYs

This patch makes changes in preparation for supporting open firmware
device tree descriptions of MDIO busses.  Changes include:
- Cleanup handling of phy_map[] entries; they are already NULLed when
  registering and so don't need to be re-cleared, and it is good practice
  to clear them out when unregistering.
- Split phy_device registration out into a new function so that the
  OF helpers can do two stage registration (separate allocation and
  registration steps).

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>