diag: dci: Add DCI command response support on APSS

Add support to enable clients to send command requests registered
on the Apps processor through DCI. It also handles dedicated
commands that are handled exclusively by the Apps processor.

Change-Id: I8bea5cbe9f787ef7213bb4d995bb0ab68a06dab2
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>