PCI: rcar: Add R-Car PCIe device tree bindings

This patch adds the bindings for the R-Car PCIe driver.  The driver resides
under drivers/pci/host/pcie-rcar.c

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Simon Horman <horms+renesas@verge.net.au>