mhi: core: add a state for shutdown with no link access

MHI power management states can transition from link down or
error fatal detected to a shutdown which assumes that link is
accessible when it may not be. Add a state so that no register
access are allowed during shutdown state.

CRs-Fixed: 2468495
Change-Id: I119d0ec31febaff07b0fdfb067a1a7bc5fd447b1
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>