board: phytec: common: phytec_som_detection: Add helper for PCB revision

Add helper function to read out the PCB revision of a PHYTEC SoM.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Yannic Moog <y.moog@phytec.de>
Tested-by: Yannic Moog <y.moog@phytec.de>