iommu/rockchip: Add map_sg callback for rk_iommu_ops

The iommu_dma_alloc() in iommu/dma-iommu.c calls iommu_map_sg()
that requires the callback iommu_ops .map_sg(). Adding the
default_iommu_map_sg() to Rockchip IOMMU accordingly.

Signed-off-by: Simon Xue <xxm@rock-chips.com>
Signed-off-by: Shunqian Zheng <xxm@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>