cmake: DT: Change from -Idts/common to -isystem dts/common

All DT include directories are system directories, except for
dts/common. There is no reason for dts/common to diverge, so we align
it with the other directories to be consistent.

Also, we move it closer to the other include directories.

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