iommu/io-pgtable-arm-v7s: Add paddr_to_iopte and iopte_to_paddr helpers

Add two helper functions: paddr_to_iopte and iopte_to_paddr.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>