ARM64: MediaTek MT8173: Add SCPSYS device node

This adds the SCPSYS device node to the MT8173 dtsi file.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>