ucsi: ucsi_glink: add SSR support

Add subsystem restart (SSR) support to UCSI Glink driver so that
it can unregister from UCSI framework when subsystem goes down
and register with UCSI framework when it comes up again. When
subsystem goes down, it is not possible for UCSI stack to send
or receive data to and from PPM (i.e. remote subsystem)  over
PMIC Glink. Hence, return immediately in ucsi_qti_{read,write}()
to avoid error logs and handle cleanup easily.

Change-Id: I711920222bd531af27b87337cad7db8aabbaa466
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>