staging: mt7621-pci-phy: dt: bindings: add mediatek, mt7621-pci-phy.yaml

Add bindings to describe Mediatek MT7621 PCIe PHY.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200410091836.13068-2-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>