drivers: dma-contiguous: add initialization from device tree 

Add device tree support for contiguous memory regions defined in device
tree. Initialization is done in 2 steps. First, the contiguous memory is
reserved, what happens very early, when only flattened device tree is
available. Then on device initialization the corresponding cma regions are
assigned to device structures.

Change-Id: Ic242499b64875ee57a346d7cbc8a34ebd64e68d2
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>