msm: subsystem_restart: Add check for platform device

While registering a subsystem, it is not necessary for the descriptor
to be associated with a platform device with entries in dt.

These changes are required for succesfully returning the registered
subsystem device in such cases.

Change-Id: Ia31234c9c216fc0b6e997e3c8661cff6edab2759
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>