cpu/arm/k60: Add macro for debugger breakpoints.

The new macro is named DEBUGGER_BREAK(sig) and takes a single 8-bit
integer as a parameter. When the line is reached it will signal to the
debugger to break.

This removes all asm("bkpt #x") calls in various drivers and replaces
them with the new macro.

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