ext: nordic: Correct CMakeLists.txt

This corrects two things:
- moves addition of the proper "system_*.c" file to sources, so that it
  is done only when CONFIG_HAS_NRFX is active (related CONFIG_SOC_* may
  be activated also for simulated targets which do not want to compile
  code with system initialization for the real SoC)
- removes redundant check of HAS_NRFX when adding "nrfx_glue.c"

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>