soc: imx8m: Introduce soc_uid hook

Cleanup code by introducing soc_uid hook, i.MX8MQ/M/N could reuse
one function imx8m_soc_uid, i.MX8MP could have its own one.

With this patch, it will easy to add 128bits UID support for i.MX8MP.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>