usb: hcd: Initialize USB phy if needed

This adds external USB phy support to USB HCD driver that
allows to find and initialize external USB phy, bound to
the HCD, when the HCD is added.
The usb_add_hcd function returns -EPROBE_DEFER if the USB
phy, bound to the HCD, is not ready.
If no USB phy is bound, the HCD is initialized as usual.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>