board: phytec: fix link error when disabling PHYTEC_SOM_DETECTION

Commit aa7858fe ("board: phytec: som_detection: move definitions to
source file") moved function definitions from header to source file.
Makefile however was not updated to unconditionally (from
[..]_SOM_DETECTION) build the imx8 and phytec som detection units.
Also remove unused includes that cause build failures on arm 32bit
boards. SoM detection shall support 32bit boards as well, but arch
specific code should not be included in the generic module.

Fixes: aa7858fe ("board: phytec: som_detection: move definitions to source file")
Signed-off-by: Yannic Moog <y.moog@phytec.de>