ARM: dts: meson8b: add power domain controller

The Meson8b SoCs have a power domain controller which can turn on/off
various register areas (such as: Ethernet, VPU, etc.).
Add the main "pwrc" controller and configure the Ethernet power domain.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20200620161010.23171-4-martin.blumenstingl@googlemail.com