ASoC: davinci-mcbsp: add binding for McBSP

Add devicetree binding for the TI DA850/OMAP-L138/AM18xx
MultiChannel Buffered Serial Port (McBSP)

The optional register range "dat" is not implemented at the moment.
The current driver supports only DMA into RX/TX registers but no FIFO.
Once the FIFO is implemented in the driver the "dat" range will be used.

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