doc: switch from CPP to C domain for .c files
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 <382671175562e0df8128054dbdb05a250d79f588@nordicsemi.no>

galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr