arm64: rockchip: Add GRF driver

RockChip GRF (General Register Files) is present on almost all RockChip
SoC and is used to control some area of the system like iomuxing, gpio
or usb phy.
We need it to be probed and attached early in the boot process so
subclass syscon_generic and set the pass to BUS_PASS_BUS + BUS_PASS_ORDER_MIDDLE.