arm64: fix build error if DMA_CMA is enabled

arm64/include/asm/dma-contiguous.h is trying to include
<asm-genric/dma-contiguous.h> which does not exist, and thus failing
build for arm64 if we enable CONFIG_DMA_CMA. This patch fixes build
error by removing unwanted header inclusion from arm64's dma-contiguous.h.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Somraj Mani <somraj.mani@samsung.com>
Acked-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>