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

Add DT bindings for PCIe controller implemented in UniPhier SoCs when
configured in Root Complex (host) mode. This controller is based on
the DesignWare PCIe core.

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>