msm: ocmem: Change the return type of the notifier api.

The ocmem notifier API currently works with a void* pointer
for registration and de-registration. Change it to return
a pointer to struct ocmem_notifier which is useful for debug
purposes.

Change-Id: I93af1213624e5da6993514078451f0474af2c89e
Signed-off-by: Naveen Ramaraj <nramaraj@codeaurora.org>