soc: imx8m: Cleanup with adding imx8m_soc_[un]prepare

There is a common flow to i.MX8M family, map OCOTP register base and
enable ocotp clk first before read Unique ID from OCOTP.

So introduce imx8m_soc_prepare to do ioremap and enable the ocotp clk,
and introduce imx8m_soc_unprepare to disable the clk and do iounmap.

With this patch, no need to spread the ioremap and clk handling in
each soc_revision hook.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>