cmake: Move MDK-defines from 'soc' to the MDK build script

The build script ext/hal/nordic/CMakeLists.txt manages the MDK
dependency. The MDK is not aware of Zephyr and uses it's own defines
to indicate what HW we are building for.

This commit moves these MDK-specific defines out of the 'soc' build
scripts and into the MDK build script where they belong.

This is expected to be a pure refactoring without side-effects.

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