dt-bindings: Amlogic: Add Meson8 and Meson8b SMP related documentation

With this patch we add documentation for:

* power-management-unit: the PMU is used to bring up the cores during
  SMP operations
* sram: among other things the sram is used to store the first code
  executed by the core when it is powered up
* cpu-enable-method: the CPU enable method used by Amlogic Meson8 and
  Meson8b SoCs

Signed-off-by: Carlo Caione <carlo@endlessm.com>
[also add Meson8 to the documentation]
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>

Signed-off-by: Kevin Hilman <khilman@baylibre.com>