clk: imx: Support building i.MX common clock driver as module

clk: imx: Support building i.MX common clock driver as module

There are more and more requirements of building SoC specific drivers
as modules, add support for building i.MX common clock driver as module
to meet the requirement.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>