msm: mdm_common: Use crash_shutdown op instead of panic notifier

Almost all PIL subsystem drivers do what the mdm_panic_prep does
in their corresponding crash_shutdown operation. This operation is
supported by SSR and moving out the panic notifier to use
crash_shutdown helps mdm to tap into this functionality.

Change-Id: I77349fea1d27d3abeb13ed546a5be3a32106dfed
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>