samplefmt: Add doxygen categories

Categorize the enum and functions as "audio-related".

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Diego Biurrun <diego@biurrun.de>
audio_fifo: Split into a separate doxygen module

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Diego Biurrun <diego@biurrun.de>