davinci: add generic CPUFreq driver for DaVinci

Adds a basic CPUFreq driver for DaVinci devices registering with the
kernel CPUFreq infrastructure.

Support is added for both frequency and voltage regulation.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>