ARM: dts: berlin: add SMP related nodes and properties for BG2

Add required nodes and properties into the Berlin BG2 device tree to
take advantage of the newly introduced SMP support. Add the scu and
cpu-ctrl nodes along with the CPUs enable-method property.

Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>