PCI: rockchip: Separate common code from RC driver

In preparation for introducing EP driver for Rockchip PCIe controller,
rename the RC driver from pcie-rockchip.c to pcie-rockchip-host.c, and
only leave some common functions in pcie-rockchip.c in order to be
reused for both of RC driver and EP driver.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Tested-by: Jeffy Chen <jeffy.chen@rock-chips.com>