iommu/rockchip: Enable Rockchip IOMMU on ARM64

This patch makes it possible to compile the rockchip-iommu driver on
ARM64, so that it can be used with 64-bit SoCs equipped with this type
of IOMMU.

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