soc: qcom: debugfs-qmp: Add a QMP client to send messages using debugfs

Add support for a debugfs node that would allow clients to communicate
with the Always On Processor(AOP) using the QMP mailbox. Clients can
write the message to be sent directly into the nodes at
<debugfs>/aop_send_message.

Change-Id: If8d51cfd92a231b6d8a540c895f56290aa7a7342
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>