staging: mt7621-pci: dt-bindings: add dt bindings for mt7621 pcie controller

This commit adds pci device tree bindings for the Mt7621 pci controller.
This is a temporal file included in staging driver directory and will be
moved to its correct location when this driver gets out of staging.

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