iommu/mediatek: Fix VLD_PA_RNG register backup when suspend

The register VLD_PA_RNG(0x118) was forgot to backup while adding 4GB
mode support for mt2712. this patch add it.

Fixes: 30e2fccf ("iommu/mediatek: Enlarge the validate PA range
for 4GB mode")
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>