iommu/mediatek: Fix two IOMMU share pagetable issue

Prepare for mt8188 to fix a two IOMMU HWs share pagetable issue.

We have two MM IOMMU HWs in mt8188, one is VPP-IOMMU, the other is
VDO-IOMMU. The 2 MM IOMMU HWs share pagetable don't work in this case:
 a) VPP-IOMMU probe firstly.
 b) VDO-IOMMU probe.
 c) The master for VDO-IOMMU probe (means frstdata is vpp-iommu).
 d) The master in another domain probe. No matter it is vdo or vpp.
Then it still create a new pagetable in step d). The problem is
"frstdata->bank[0]->m4u_dom" was not initialized. Then when d) enter, it
still create a new one.

In this patch, we create a new variable "share_dom" for this share
pgtable case, it should be helpful for readable. and put all the share
pgtable logic in the mtk_iommu_domain_finalise.

In mt8195, the master of VPP-IOMMU probes before than VDO-IOMMU
from its dtsi node sequence, we don't see this issue in it. Prepare for
mt8188.

Fixes: 645b87c1

 ("iommu/mediatek: Fix 2 HW sharing pgtable issue")
Signed-off-by: Chengci.Xu <chengci.xu@mediatek.com>
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://lore.kernel.org/r/20230602090227.7264-3-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>