ASoC: davinci-mcasp: Support for one channel (mono) audio

Mono audio can be achieved by configuring McASP to transmit/receive only
during one timeslot. McASP will still going to generate clocks for the
other slot(s), but will only use the single slot to transmit/receive.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>