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.

Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>