clk: imx: scu: add gpr clocks support

SCU clock protocol supports a few clocks based on GPR controller
registers including mux/divider/gate.
Add a generic clock register API to support them all.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>