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 110d321a56c3 ("board: phytec: common: phytec_som_detection: Add phytec_get_som_type")

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>
diff --git a/board/phytec/common/phytec_som_detection.c b/board/phytec/common/phytec_som_detection.c
index f9607b0..a56e0f6 100644
--- a/board/phytec/common/phytec_som_detection.c
+++ b/board/phytec/common/phytec_som_detection.c
@@ -248,4 +248,9 @@
 	return PHYTEC_EEPROM_INVAL;
 }
 
+u8 __maybe_unused phytec_get_som_type(struct phytec_eeprom_data *data)
+{
+	return PHYTEC_EEPROM_INVAL;
+}
+
 #endif /* IS_ENABLED(CONFIG_PHYTEC_SOM_DETECTION) */