slimbus: Support multiple message transactions per call

Bandwidth intensive operations (such as firmware download) need
lot of messages to be sent over the bus. This API adds support to
queue multiple messages to HW and provides controller implementation
using BAM.
The API can also be used in places where slimbus slaves have strict
ordering/timing restrictions (e.g. send 2 specific register-writes
back-to-back).

Change-Id: I9a0779e0c4b957d4f4e3b31567b983457d367193
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>