arm64: dts: mt8192: Add spmi node

Add spmi node for mt8192 SoC.

Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Link: https://lore.kernel.org/r/20220419063226.15958-2-allen-kh.cheng@mediatek.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>