cma: Return 0 on error path

If a free CMA region cannot be found because every one is busy,
an error needs to be propegated up by returning a zero pfn.
Ensure the pfn is actually zero when returning an error.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>

Conflicts:
	drivers/base/dma-contiguous.c
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.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>