cmake: cleanup search for devicetree compiler, dtc

Cleanup CMake code by moving devicetree compiler detection to dedicated
FindDtc.cmake module.

This allows us to re-use existing CMake find_package() functionality in
module mode and thus reduce CMake logic.

It further has the benefit of being able to directly specify dtc version
like this:
> find_package(Dtc 1.4.6)

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