cpu/arm/k60: Move linker script to separate directory

This is in preparation of more linker scripts.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Modularise ldscripts

* Split ldscript into separate files for inclusion in part specific
  script.

* Add ldscript for K60DN512VLL10 (512KB flash, 64KB ram) parts.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>