arm64: dts: mt8173: Fix mdp device tree

If the mdp_* nodes are under an mdp sub-node, their corresponding
platform device does not automatically get its iommu assigned properly.

Fix this by moving the mdp component nodes up a level such that they are
siblings of mdp and all other SoC subsystems.  This also simplifies the
device tree.

Although it fixes iommu assignment issue, it also break compatibility
with old device tree. So, the patch in driver is needed to iterate over
sibling mdp device nodes, not child ones, to keep driver work properly.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Minghsiu Tsai <minghsiu.tsai@mediatek.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>