OMAP: Introduce voltage domain information in the hwmod structures

This patch extends the device hwmod structure to contain
info about the voltage domain to which the device belongs to.
This is needed to support a device based DVFS where the
device knows which voltage domain it belongs to.

Signed-off-by: Thara Gopinath <thara@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>