cma: Add support for associating regions by name 

Currently, the devicetree lookup code assumes that all
CMA regions are present at a fixed address and uses the
fixed address for associating CMA regions to devices.
This presents a problem for dynamically assigning regions.
Device names get mangled/changed between flattened and
populated devicetree so relying on that is unworkable.
Add a separate name binding to allow lookup later between
devices.

Change-Id: Iaacd9888ea708d7293f1120e2b8c473c5c601f3d
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>