ARM: dts: meson8b: add support for booting the secondary CPU cores

Booting the secondary CPU cores involves the following nodes/devices:
- SCU (Snoop-Control-Unit, for which we already have a DT node)
- a reset line for each CPU core, provided by the reset-controller
  which is built into the clock-controller
- the PMU (power management unit) which controls the power of the CPU
  cores
- a range in the SRAM specifically reserved for booting secondary CPU
  cores
- the "enable-method" which activates booting the secondary CPU cores

This adds all required nodes and properties to boot the secondary CPU
cores.

Signed-off-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>