davinci: DM646x: add base SoC and board support

Add support for DM646x SoC (a.k.a DaVinci HD) and its Evalution
Module (EVM.)

Original support done by Sudhakar Rajashekhara.

Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>