arm64: dts: mt6797: Fix mmsys node name

Node names are supposed to match the class of the device. The
mmsys node is a syscon as it provides more then just a clock controller.
Update the name.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/20200518113156.25009-4-matthias.bgg@kernel.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>