UID information in OCOTP is located in the same place on at least
i.MX6, i.MX7 and i.MX8MQ. Create an SoC-agnostic helper to do all the
work and convert i.MX6 to use it.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>