dma-direct: add dependencies to CONFIG_DMA_GLOBAL_POOL

CONFIG_DMA_GLOBAL_POOL can't be combined with other DMA coherent
allocators.  Add dependencies to Kconfig to document this, and make
kconfig complain about unmet dependencies if someone tries.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>