phy: helpers for USB ULPI PHY registering

ULPI PHYs need to be bound to their controllers with a
lookup. This adds helpers that the ULPI drivers can use to
do both, the registration of the PHY and the lookup, at the
same time.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: David Cohen <david.a.cohen@linux.intel.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>