platform/mulle/lis3dh: Clean up header.

Remove lis3dh_{enable,disable}_adc() which have been replaced by
lis3dh_set_aux_adc().
Add declarations of lis3dh_read_aux_adc{1,2,3} for reading the
auxiliary ADC values.

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