ARM: dts: meson8b: add the SCU device node

Amlogic's Meson8b SoC has a Snoop Control Unit (SCU), just like many
other Cortex-A5 SoCs. Add the corresponding devicetree node so it can be
used during SMP boot.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>