dts: edtlib: Add simple generic support for pinctrl-<index> properties

(pinctrl-<index> is documented in
Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt in
Linux.)

Add a new Node.pin_states property, derived from any pinctrl-<index>
properties on the node. Node.pin_states holds a list of PinState
objects, where each PinState represents a single pinctrl-<index>
property.

For example, Node.pin_states will have two elements for the 'device'
node below:

	device {
		pinctrl-0 = <&state_0>;
		pinctrl-1 = <&state_1 &state_2>;
		pinctrl-names = "default", "sleep";
	};
	pincontroller {
		state_0: state_0 {
			...
		};
		state_1: state_1 {
			...
		};
		state_2: state_2 {
			...
		};
	};

Each PinState holds the list of configurations nodes in
PinState.conf_nodes. For the node above, node.pin_states[1].conf_nodes
will contain the pincontroller/state_1 and pincontroller/state_2 nodes,
for example.

The new functionality isn't used by gen_defines.py yet, so this change
is a no-op in itself, except it adds some error checking for
pinctrl-<index> properties.

If needed, support for #pinctrl-cells and 'pinmux' (not the same thing
as the 'pinmux' properties in Zephyr I think) could be added separately
later. Not sure what belongs in edtlib.py there yet.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>