doc: switch from CPP to C domain for .c files

Older Breathe versions didn't work correctly with the C domain, so there
was a mismatch when refering to functions and #defines, using :cpp: or
:c: depending on the origin of the refered symbol. With Breathe >=4.19
the C domain works correctly so all symbols can be refered to with the
:c: role.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>