clk: imx8mm: Switch to platform driver

There is no strong reason for this to use CLK_OF_DECLARE instead
of being a platform driver. Plus, this will now be aligned with the
other i.MX8M clock drivers which are platform drivers.

In order to make the clock provider a platform driver
all the data and code needs to be outside of .init section.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>