soc: imx: add i.MX51/i.MX53 unique id support

i.MX51 and i.MX53 SoCs have a 64-bit SoC unique ID stored in IIM,
which can be used as SoC serial number. The same feature is already
implemented for i.MX6/i.MX7, so this complements support to earlier
SoCs.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>