cma: use pfn instead of pages for argument passing

The CMA code is generic enough that it can be expanded out to track
regions of memory that aren't officially managed by the regular page
allocator. This memory can't be referenced via struct page. Change the
CMA apis to track using pfn for allocation/free instead. The pfn can
be converted to a struct page as needed elsewhere.

Change-Id: I5ac3fa5e2169b2101a738177f1654faa401f7604
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>