UPSTREAM: clk: rockchip: add a clock-type for muxes based in the grf

UPSTREAM: clk: rockchip: add a clock-type for muxes based in the grf

Rockchip socs often have some tiny number of muxes not controlled from
the core clock controller but through bits set in the general register
files. Add a clock-type that can control these as well, so that we
don't need to work around them being absent.

BUG=None
TEST=Build and boot on RK3399

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
(cherry picked from git.kernel.org mmind/linux-rockchip.git v4.11-clk/next
 commit cb1d9f6ddaa436f2dce2710740b7a3546700949c)

Change-Id: I79352c8596f4d03fde519cd544c9d509d84c3a66
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>