cpu/arm/k60: Add switchable support for silicon version 1.x

The file config-board.h shall define K60_CPU_REV which can be
either 1 or 2 to switch between the different CPU headers.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add clock initialization code.

This is a simple implementation of using the RTC 32k crystal
for clocking the FLL.

The clocks can be configured in config-clocks.h in the platform
directory, however, there are still several hard coded delay
constants that will be off if the frequency is changed.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
examples/mulle-clocks: Simple clock debug helper.

Useful for inspecting the clocks with an oscilloscope.

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