clk: imx8m: Support module build

Change configuration to "tristate", add module author, description
and license to support building i.MX8M SoCs clock driver as module.

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