soc: imx8: Add i.MX8MM UID(unique identifier) support

Add i.MX8MM SoC UID(unique identifier) support, user
can read it from sysfs:

root@imx8mmevk:~# cat /sys/devices/soc0/soc_uid
B365FA0A5C85D6EE

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