esoc: mdm-dbg: Add a debug request engine for external mdm

Add a stub request engine for debugging external modem.

Change-Id: I6360831f21f83fcd25df72492b1d57b30a7d2197
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>
Signed-off-by: Bruce Levy <blevy@codeaurora.org>
[satyap@codeaurora.org: fix checkpatch issue]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>