soc: imx8: Add i.MX8MQ UID(unique identifier) support

Add i.MX8MQ SoC UID(unique identifier) support, user
can read it from sysfs:

root@imx8mqevk:~# cat /sys/devices/soc0/soc_uid
D56911D6F060954B

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