msm: gsi: add support for generic commands

Expose a new API from GSI driver to allow client driver to
disable channel for other Execution Environment.
This API will be used as part of SSR cleanup.

Change-Id: I3b9400643aff76ca2195a597aba9ea18aab3085e
CRs-Fixed: 2008582
Acked-by: Ady Abrahan <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>