soc: imx: Add generic i.MX8 SoC driver

Add generic i.MX8 SoC driver along with the i.MX8MQ SoC specific code.
For now, only i.MX8MQ revision B1 is supported. For any other, i.MX8MQ
revision it will print 'unknown'.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>