cmake: cleanup and simplify the standard include logic in Zephyr

[tejlmand]

Several paths are checked for existence before added as global Zephyr include path.

The existence check was needed because some tooling emit warnings on non-existing paths.

Only few SoCs are using those pre-defined paths, yet this code runs for all SoCs. The principle originates back from Kbuild days, and with CMake it's more common and generally more visible to let the CMake code defining libraries to specify include paths.

Furthermore it appears that several SoC implementation following the <soc-path>/include was unaware that the path would be automatically added as include path, cause they contain lines like:

    zephyr_library_include_directories(include)

Remove pre-defineds path except the <SOC_PATH> path, which is guaranteed to exists.
This simplifies the CMake logic in the top-level Zephyr CMakeLists.txt file.

This cleanup further prepares for future work where SoCs need not to be organised under architectures which is important for multi-arch SoCs.