power: domain: Add the VPU Power Domain driver

power: domain: Add the VPU Power Domain driver

The Amlogic Meson SoCs embeds a specific Power Domain dedicated to the
Video Processing Unit.
This patch implements support for this power domain in preparation of the
future support for the Video display support in U-Boot.

This driver will depend on changes in the clock driver to handle the setup
of the VPU and VAPB clocks configured from DT using assigned-clocks entries.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>