Add device tree binding documentation for PCIe in MT7621 SoCs.

Link: https://lore.kernel.org/r/20210922050035.18162-2-sergio.paracuellos@gmail.comSigned-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>