ASoC: Add mcasp support for DM646x

Adds driver support for the two instances of McASP on TI's DM646x.

The multichannel audio serial port (McASP) functions as a general-purpose audio
serial port optimized for the needs of multichannel audio application.
(http://www.ti.com/litv/pdf/spruer1b

).

There are two instances of McASP on DM646x. The McASP0 module includes up to 4
serializers that can be individually enabled to either transmit or receive
in different modes. The McASP1 module is limited with only 1 pinned-out
serializer that can be enabled to only transmit in DIT mode (neither receiving
in any mode nor transmitting in either Burst or TDM mode is supported).

McASP0 consists of transmit and receive sections that may operate
synchronized, or completely independently with separate master clocks, bit
clocks, and frame syncs, and using different transmit modes with different
bit-stream formats.

Signed-off-by: Steve Chen <schen@mvista.com>
Signed-off-by: Pavel Kiryukhin <pkiryukhin@ru.mvista.com>
Signed-off-by: Naresh Medisetty <naresh@ti.com>
Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>