diag: Add sub-system restart support for peripherals

DIAG char driver maintains registrations for remote processors in a table.
When the remote processor goes down, the registrations should be cleaned
up. This change adds the clean up process in event of a restart. Also when
the peripheral goes down for subsystem restart, the SMD channel between
apps processor and peripheral has to be set to NULL, to prevent further SMD
operations. When the peripheral comes back up, this SMD channel can be
re-initialized.

CRs-Fixed: 326120
Change-Id: I49afad5e3bc04f62024c1dc66f272726ae13c3e6
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>