dt-bindings: soc: rockchip: add rockchip-io-domain.yaml object to grf.yaml

The rockchip-io-domain.txt file was converted to YAML.
IO Voltage Domain nodes are sub nodes of grf and pmugrf nodes.
Add rockchip-io-domain.yaml object to the grf.yaml file.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210625111746.6269-2-jbx6244@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>