c6x: Provide dummy dma_mmap_coherent() and dma_get_sgtable()

c6x: Provide dummy dma_mmap_coherent() and dma_get_sgtable()

c6x/allmodconfig (assumed):

drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_mmap’:
drivers/media/v4l2-core/videobuf2-dma-contig.c:204: error: implicit declaration of function ‘dma_mmap_coherent’
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_mmap_coherent() and
dma_get_sgtable() are provided in <asm-generic/dma-mapping-common.h>.

C6x does not use dma_map_ops, hence it should implement them itself.
For now, use dummy implementations that just return -EINVAL, until the API
has been finalized.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: linux-c6x-dev@linux-c6x.org
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>