mm: add zone counter for cma pages

Add per free area nr_free_cma counter. The idea is
to also track the number of cma pages present in
free pages. This will be used in later patches to
fix issues with zone_watermark_ok.

Change-Id: I97da9d2f3642db56fc541c48ab56a7ce78e2333c
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>