arm64: dts: meson-axg: add PWRC node

This adds the power controller PWRC node and the corresponding ethernet power domain.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20201120152131.3918814-2-narmstrong@baylibre.com