metag: Provide dma_get_sgtable()

metag/allmodconfig:

drivers/media/v4l2-core/videobuf2-dma-contig.c: In function 'vb2_dc_get_base_sgt':
drivers/media/v4l2-core/videobuf2-dma-contig.c:387: error: implicit declaration of function 'dma_get_sgtable'

For architectures using dma_map_ops, dma_get_sgtable() is provided in
<asm-generic/dma-mapping-common.h>.

Metag does not use dma_map_ops yet, hence it should implement it as an
inline stub using dma_common_get_sgtable().

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>