msm: mhi_dev: Add callback support for kernel clients

Add a callback for kernel clients to receive notification
once the software channel for which the client has registered
has opened/closed.

Change-Id: Ie97e71b5c9e007a659d0f33ac0bf110327494e9e
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>