conf/ucm: bytcr-rt5651: Document mono speaker wiring

conf/ucm: bytcr-rt5651: Document mono speaker wiring

During my recent work on the bytcht-es8316 UCM profile I realized that the
bytcr-rt5651 devices with a single speaker use a differential setup just
like the es8316 does. The tell-tale here is the speaker going quiet when
playing the exact same sound on both channels when things are configured
for stereo speakers.

I've run some tests and the rt5651 does have a special mono balanced out
mode for its line-out but using this results in the same sound (and the
same loudness / volume of the sound) as our current solution, so adding
support for this to the kernel buys us nothing.

This commit makes no changes, it just documents my findings in a big
comment for future reference.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>