platform/mulle: Add comments to slip_arch_init.

Added comments and todo for later reference.

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

Replaced magic numbers with symbolic macros.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add more debug output during boot.

 - Adds a printout for system frequencies.
 - Replaced printf() with PRINTF() to match other parts of the file.

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