iommu/mediatek: Add a new tlb_lock for tlb_flush

The commit 4d689b61 ("iommu/io-pgtable-arm-v7s: Convert to IOMMU API
TLB sync") help move the tlb_sync of unmap from v7s into the iommu
framework. It helps add a new function "mtk_iommu_iotlb_sync", But it
lacked the lock, then it will cause the variable "tlb_flush_active"
may be changed unexpectedly, we could see this warning log randomly:

mtk-iommu 10205000.iommu: Partial TLB flush timed out, falling back to
full flush

The HW requires tlb_flush/tlb_sync in pairs strictly, this patch adds
a new tlb_lock for tlb operations to fix this issue.

Fixes: 4d689b61 ("iommu/io-pgtable-arm-v7s: Convert to IOMMU API TLB sync")
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>