Add support to detect i.MX6q vs i.MX6dl variants

Rewrite the platform detection, use machine and soc_id from
/sys/device/soc0 as first attempt, fallback to gfplatdetect and
proc_cpuinfo if needed.
Soc_id is an array in the platform struct that has to map to the
read soc_id to select a platform.

Also added pinfunc headers to use macros for PAD-CTL.

BCS 746-000417