clk: imx: imx8mn: Switch to clk_hw based API

Switch the entire clk-imx8mn driver to clk_hw based API.
This allows us to move closer to a clear split between
consumer and provider clk APIs.
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>