AVR32: Fix sg_page breakage

The latest sg changes introduce the following build errors on AVR32:

include/asm/dma-mapping.h: In function ‘dma_map_sg’:
include/asm/dma-mapping.h:220: error: implicit declaration of function ‘sg_page’
include/asm/dma-mapping.h:220: error: invalid operands to binary -
include/asm/dma-mapping.h:221: error: implicit declaration of function ‘sg_virt’
include/asm/dma-mapping.h:221: warning: assignment makes pointer from integer without a cast
include/asm/dma-mapping.h: In function ‘dma_sync_sg_for_device’:
include/asm/dma-mapping.h:330: warning: passing argument 2 of ‘dma_cache_sync’ makes pointer from integer without a cast

Fix it by including the correct header file, i.e. linux/scatterlist.h
instead of asm/scatterlist.h.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>