dt-bindings: power: rockchip: add regulator support

Add optional support for a voltage supply required to enable a
power domain. The binding follows the way it is handled by the
Mediatek binding to keep things consistent.

This will initially be used by the RK3588 GPU power domain, which
fails to be enabled when the GPU regulator is not enabled. It is
not limited to that platform, since older generations have similar
requirements. They worked around this by marking the regulators
as always-on instead of describing the dependency.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>