soc: imx: Add i.MX8MN SoC driver support

This patch adds i.MX8MN SoC driver support:

root@imx8mnevk:~# cat /sys/devices/soc0/family
Freescale i.MX

root@imx8mnevk:~# cat /sys/devices/soc0/machine
NXP i.MX8MNano DDR4 EVK board

root@imx8mnevk:~# cat /sys/devices/soc0/soc_id
i.MX8MN

root@imx8mnevk:~# cat /sys/devices/soc0/revision
1.0

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>