pinctrl: rockchip: add support for the rk3368

The rk3368 is the first ARM64 soc from Rockchip, but seems to share most
peripherals with the ARM32 soc, including the pinctrl functionality.
The only notable difference is - as with every Rockchip soc - that the
offsets in the General Register Files moved around and a split of the pmu
section of the rk3288 into pmu and pmugrf (pmu general register files)
sections. The pinctrl driver of course only needs the pmugrf registers
for controlling the pin settings.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>