iommu/mediatek: Fix driver name

There exist two Mediatek iommu drivers for the two different
generations of the device. But both drivers have the same name
"mtk-iommu". This breaks the registration of the second driver:

Error: Driver 'mtk-iommu' is already registered, aborting...

Fix this by changing the name for first generation to
"mtk-iommu-v1".

Fixes: b17336c5 ("iommu/mediatek: add support for mtk iommu generation one HW")
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>