mhi: core: keep MHI in M0 while entering suspend

During MHI suspend, host must keep MHI wake asserted to prevent
device from entering any other low power states.

CRs-Fixed: 2361072
Change-Id: Ib68ec62294eab4727c6bb8e356b6aa59b9ca99cf
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>