msm: msm_bus: Add proxy bus client driver

Add a proxy client driver which can be used to cast a proxy
bandwidth vote during boot up for a critical resource which
is removed at the end of boot. This is required to ensure
that a certain minimum state for a critical resource at the
beginning of boot up before the clients have probed and voted
for required state.

Change-Id: I34638c458604231a68b926bbe5110dcf96162352
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>