msm: camera: isp: Create union for update command payload

The update command can take different types of commands, new commands
may have a different structure requirements. Create union for
the payload so new commands can be added with their own payload
structure.

Change-Id: Ia43e47f13c112122baec87ee7b639c17b49366c4
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>
Signed-off-by: Harsh Shah <harshs@codeaurora.org>