staging: mt7621-pci-phy: dt-bindings: add bindings for Mediatek MT7621 Pcie PHY

Add bindings documentation for PCie PHY of Mediatek MT7621.
This file will be moved into its appropiate documentation bindings'place
when this driver is mainlined.

CC: Device Tree mailing list <devicetree@vger.kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>