board: phytec: define get_som_type also when SoM detection is disabled

define the phytec_get_som_type function also when the SoM detection is
disabled.

Fixes:
commit 110d321a ("board: phytec: common: phytec_som_detection: Add phytec_get_som_type")

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>