mhi: core: allow communication during shutdown

MHI clients allowed to communicate with external
soc via MHI during shutdown. Only disable MHI
communication after receiving shutdown notification.

CRs-Fixed: 2037184
Change-Id: If69f61a244a6175256753dea361085fa5f9afeac
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>