ARC: don't check for HIGHMEM pages in arch_dma_alloc

__GFP_HIGHMEM flag is cleared by upper layer functions
(in include/linux/dma-mapping.h) so we'll never get a
__GFP_HIGHMEM flag in arch_dma_alloc gfp argument.
That's why alloc_pages will never return highmem page
here.

Get rid of highmem pages handling and cleanup arch_dma_alloc
and arch_dma_free functions.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>