PM / AVS: rockchip-io: make io-domains a child of the GRF

IO-domain handling is part of the general register files, so should live
under the grf directly. This change allows the grf to be a simple-mfd and
the io-domains fetching the syscon regmap from that parent-node.

The old binding is of course preserved, though deprecated.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>