davinci: dm646x EVM: Specify reserved EDMA channel/slots

Not all the channels and slots available on the DM646x EVM
are used by the devices on the EVM. These resources can be
used by the DSP to speed up codec operations.

This patch reserves these channels for the DSP.

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