soc: imx8m: Support building imx8m soc driver as module

Make the imx8m soc driver as tristate so that it can be built as loadable
module, module names are:
soc-imx8m.ko, imx8m-blk-ctrl.ko and imx8mp-blk-ctrl.ko

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