imx: kontron-sl-mx6ul: Add support for reading HW UIDs

imx: kontron-sl-mx6ul: Add support for reading HW UIDs

The factory provides a CPU UID in the OTPs and the SoM module
and the carrier board might provide additional UIDs in the GP
registers of the OTPs. Load these values in the following order
and create a serial number string:

* Board UID (GP2)
* SoM UID (GP1)
* CPU UID (UNIQUE_ID)

The string is stored in the "serial#" env variable and
exported to Linux. Further this prints the used UID to the
console which looks like this

  ID:    0042152331 (Board)

or:

  ID:    0030124840 (SoM)

or:

  ID:    4696668CD9516886 (CPU)

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>