msm: subsys-restart: Convert subsys devices from misc to char

Using misc devices causes lock contention because there is only
one lock for all misc devices. Instead, model the subsys devices
as charecter devices.
The subsys charecter devices can be now accesed by opening/closing
the corresponding /dev/subsys_* files.

Change-Id: I4afcb5cccba23b59e4b3849f7d0a5945edfa9d9e
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>