ASoC: davinci-mcasp: Remove redundant num-serializer DT parameter

The serial-dir array gives this information so there is no need to have the
num-serializer property in DT description.
Just ignore the property in the driver the DTS files can be updated
separately without regression.
Update the documentation at the same time for davinci-mcasp

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>