dts: bindings: pinctrl: place pincfg-node props at root level

Since we can include other binding files at any level (child,
grand-child, etc.) it makes no sense to maintain two copies of pinctrl
props definitions (pincfg-node/pincfg-node-group). Instead,
pincfg-node.yaml defines props at root level, and it is included where
needed, either child-binding or grandchild-binding.

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