ASoC: davinci-mcasp: Improve DT bindings document

Makes interrupts property optional as the interrupts are not currently
used by the driver and adds interrupt-names property to name listed
interrupts. Currently know interrupt names are "tx" and "rx".

- Improve tdm-slots propery description

- Improve op-mode property description

- Add pinctrl-names and pinctrl-0 properties

- Remove #address-cells and #size-cells as they are not needed.

- Bracket named interrupts property tuples for uniformity.

- Add missing "for" to serial-dir prop in DT bindings doc.

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