Documentation: bindings: add dt documentation for Rockchip PCIe PHY

This patch adds a binding that describes the Rockchip PCIe PHY found
on Rockchip SoCs PCIe interface.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>