slimbus: Enable clients to send user messages to devices.

Slimbus protocol allows user messages of destination referred, or
source referred type to be sent to a slimbus device.
Provide framework to support such messages. Note that, controller
may not support all user messages.

CRs-Fixed: 503461
Change-Id: I3644c6d671a620122095ace64da64c4e71eb9607
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>