net: davinci_emac: separate out cpdma code

In addition to being embedded into the EMAC controller, the CPDMA hardware
block is used in TI's CPSW switch controller.  Fortunately, the programming
interface to this hardware block remains pretty nicely consistent across these
devices.

This patch adds a new CPDMA services layer, which can then be reused across
EMAC and CPSW drivers.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Tested-by: Michael Williamson <michael.williamson@criticallink.com>
Tested-by: Caglar Akyuz <caglarakyuz@gmail.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>