dts: bindings: pinctrl: add pincfg-node-group

When using group based representation on pinctrl nodes, the pin
configuration properties end up being at the grand-children level, so
the `pincfg-node.yaml` file can't be used.

Having a common file that can be used for both cases would require
tooling changes, so for now a copy that operated at the grand-children
level has been created.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>