arm64/dma-mapping: __generic_dma_ops always call swiotlb_dma_ops

arm64/dma-mapping: __generic_dma_ops always call swiotlb_dma_ops

Arm64 dma-mapping will return dummy_dma_ops if none call arch_setup_dma_ops
at device creation time.

For ion APIS, ion_pages_sync_for_device and ion_pages_sync_for_cpu always
pass "NULL" as device, then dma-mapping return dummy_dma_ops, which cause
ion pages fail to sync.

This patch fixs __generic_dma_ops to return swiotlb_dma_ops instead of
dummy_dma_ops.

Change-Id: Ic00db5d4547bcd5f74e5fd155133fde408a05cdb
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>