iommu/mediatek: Get rid of the pgtlock

Now we have tlb_lock for the HW tlb flush, then pgtable code hasn't
needed the external "pgtlock" for a while. this patch remove the
"pgtlock".

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>