ARM: imx: Add serial number support for i.MX6/7 SoCs

i.MX6/7 SoCs have a 64-bit SoC unique ID stored in OCOTP,
it can be used as SoC serial number, add this support for
i.MX6Q/6DL/6SL/6SX/6SLL/6UL/6ULL/6ULZ/7D, see below example
on i.MX6Q:

root@imx6qpdlsolox:~# cat /sys/devices/soc0/serial_number
240F31D4E1FDFCA7

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