PCI: rockchip: Add EP driver for Rockchip PCIe controller

Add support for the Rockchip PCIe controller in endpoint mode;
it currently supports up to 32 regions with each region spanning
at least 1MB as per TRM.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>