soc: qcom: sideband: add helper APIs for sideband notifications

This commit introduces helper APIs and header files to be used
by sideband events publishers and subscribers.

Change-Id: I713b5b2a8dda2309453b433d922546f16a6f7e7e
Signed-off-by: Rishi Gupta <rishgupt@codeaurora.org>