cmake: dts: getting realpath for system includes for dts.

Fixes: #28893

Now using get_filename_component(REALPATH) to ensure full path with
symlinks and `..` resolved.

Having `..` in path causes the gcc generated dependency file to contain
`\` in path on windows. This confuses ninja.
The result is that ninja consider the file missing, and thus invokes a
CMake run to create the file, resulting in endless loop.

By ensuring that full path (containing `/` which is already ensured)
when calling gcc, then all paths will be posix style, making ninja
happy.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>