cpu/arm/k60: Add driver for on-CPU ADC.

The driver provides functions for reading the raw measurement as well
as calibration, however, no initialization is provided and must be
performed before calling any of the functions.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add voltage reading API.

A couple of helper functions to configure and read the onboard VBAT and
VCHR voltages. The ADC channels as well as the reference voltages can
be configured via config-board.h in the platform/mulle directory.

TODO: Make voltage_init() obey configuration (remove hard-coded channels etc.)

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
examples/mulle-voltages: Simple voltage reader for Mulle platform.

A basic example showing how to use the voltage reader functions on the
Mulle platform.

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