usb: allow to supply the PHY in the drivers when using HCD

This patch modify the generic code handling PHYs to allow them to be
supplied from the drivers. This adds checks to ensure no PHY was already
there when looking for one in the generic code. This also makes sure we
do not modify its state in the generic HCD functions, it was provided by
the driver.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>