cmake: Rebuild when DTS sources are modified
cmake: Rebuild when DTS sources are modified

Ensure that changes to DeviceTree sources cause CMake to be re-run
when make/ninja is invoked.

Note that this is not perfect, as it does not cover files that are
\#included, but it will cover most DT changes.

This fixes #12692

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

finikorg/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,nashif/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr