dt-bindings: PCI: rockchip: Rename rockchip-pcie.txt to rockchip-pcie-host.txt

Make it more obvious that this documentation is referenced for
adding the Rockchip PCIe controller as RC mode.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>