arm64: dts: ti: am654: Add Main System Control Module node

Main System control module support is added to the device tree to allow
driver to access to their control module registers.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>