xhci_mv: Move the driver to generic_xhci

Marvell XHCI is in fact generic-xhci, so move the driver and
add the compatible string.
While here, get and enable the phy if the dtb provide one.
The xhci bindings state that phys should be in a 'phys' property but
Marvell DTS uses 'usb-phy', only add support for 'usb-phy' for now.

Sponsored-by: Rubicon Communications, LCC ("Netgate")