dt-bindings: PCI: Add UniPhier PCIe endpoint controller description

Add DT bindings for PCIe controller implemented in UniPhier SoCs
when configured in endpoint mode. This controller is based on
the DesignWare PCIe core.

Link: https://lore.kernel.org/r/1589457801-12796-2-git-send-email-hayashi.kunihiko@socionext.com


Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>