ASoC: Add machine driver support for DM646x

This patch does the following:
(1) Add support for the DM646x machine
(2) Modifications required to introduce the platform driver model to get
platform data for all the machines including dm355 and dm644x.

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>