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: NCarlo Caione <carlo@endlessm.com>
[also add Meson8 to the documentation]
Signed-off-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NKevin Hilman <khilman@baylibre.com>