drivers: dma-contiguous: clean source code and prepare for device tree

This patch cleans the initialization of dma contiguous framework. The
all-in-one dma_declare_contiguous() function is now separated into
dma_contiguous_reserve_area() which only steals the the memory from
memblock allocator and dma_contiguous_add_device() function, which
assigns given device to the specified reserved memory area. This improves
the flexibility in defining contiguous memory areas and assigning device
to them, because now it is possible to assign more than one device to
the given contiguous memory area. This split in initialization is also
required for upcoming device tree support.

Change-Id: Ibddd1c9abc6550ee62b09645e7a3355256838bfe
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>