dt-bindings: pci: rcar-pci-ep: Document missing interrupts property

The R-Car PCIe controller does not use interrupts when configured
for endpoint mode, hence the bindings do not document the interrupts
property.  However, all DTS files provide interrupts properties, and
thus fail to validate.

Fix this by documenting the interrupts property.

Fixes: 4c0f8092 ("dt-bindings: PCI: rcar: Add bindings for R-Car PCIe endpoint controller")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20201209101231.2206479-1-geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>