mm: cma: Register with show_mem notification framework

Register with the show_mem notification framework
to let cma dump out data for debugging.

This is an example output with this patch.

 cma: cma-0 pages: => 0 used of 2048 total pages
 cma: cma-1 pages: => 0 used of 23552 total pages
 cma: cma-2 pages: => 1696 used of 3072 total pages
 cma: cma-3 pages: => 8277 used of 9216 total pages
 cma: cma-4 pages: => 186 used of 5120 total pages
 cma: cma-5 pages: => 3792 used of 8192 total pages.

Change-Id: I80d9291444fd9361f5586bdf60373ed8b5b41705
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>
[<swatsrid@codeaurora.org: Fix trivial errors in commit text]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>