usb: ehci-omap: Use PHY system to manage phy resets

There are a few boards that use hard-coded GPIO definitions in
their respective defconfig files.  If the GPIO's are listed
in their device trees, the nop-phy can toggle the GPIO's,
so the EHCI driver does not need to know anything about the
GPIO's. Add functions for getting the phys and remove the GPIO
toggles since the phy will now do that.

Signed-off-by: Adam Ford <aford173@gmail.com>