cma: Change to reserve-contiguous-region

Currently, devices are associated with CMA regions via phandle to the
CMA region. The phandle name 'linux,contiguous-region' is the same as
that which is used to designate a CMA region. This can lead to issues
of trying to treat a device only using a CMA region as an actual CMA
region. Rather than continuing to rely on this handle and the depth
to differentiate CMA regions, just create separate DT binding
linux,reserve-contiguous-region to indicate that the node is
an actual CMA node for reserving rather than a client referencing
via phandle.

Change-Id: I88b2d86054525b0569efc424da87974509ce9b25
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>