staging: mt7621-dts: add pci-phy related bindings to board's device tree

New driver for pci phy has been added, as well as. pci driver has been
changed to use kernel's generic PHY API. Add related PCI PHY bindings
accordly.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>