feat(mediatek): extend SiP vendor subscription events

Leverage pubsub event framework to customize vendor's
event for better software modularization instead of adding
call entries in abstraction layer for customized platform function
with wrap-up define.

Signed-off-by: Leon Chen <leon.chen@mediatek.com>
Change-Id: I48be2303c45f759776fa2baa1c21130c1a8f0fa3