Similarly as other extres pseudo-drivers, implement phy by using kobj model.
This detaches it from provider device, so single device driver can export
multiple different phys. Additionally, this allows phy to be subclassed to
more specialized drivers, like is USB OTG phy, or PCIe phy with hot-plug
capability.
Tested by: manu (previous version, on Allwinner board)
MFC after: 1 month
