cmake: only write devicetree files when there are changes.

As part of #40167 is was discovered that devicetree headers are always
generated when CMake re-runs.

This causes all source files that directly or indirectly through
included headers to recompile when CMake re-runs, even if there are no
changes to devicetree.

This commits introduces `zephyr_file_copy(...)` similar to
`file(COPY_FILE ...)` from CMake 3.21.
However, as CMake 3.20 is supported by Zephyr we need a zephyr variant
of this function to allow usage with CMake 3.20.

This ensures that only when there are changes to devicetree headers,
then source files will recompile.

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