pinctrl: rockchip: base regmap supplied by a syscon

This allows the basic registers of the general register files to be supplied
by a syscon instead of being mapped locally.

The GRF registers contain a lot more than pinctrl functions like dma, usb-phy
and general soc control and status registers, intermixed with the iomux, pull
and drive-strength registers.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Max Schwarz <max.schwarz@online.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>