scripts: gen_defines: emit pinctrl helper macros

We need to be able to access pinctrl-<index> property contents by
name, convert names to indexes, convert indexes to names, and perform
existence checks by name and by index.

This is currently not possible because we don't track these properties
the same way we do other named properties. That in turn is because
they are different then the usual named properties.

The usual case looks like this, picking DMAs just for the sake of
example:

  dmas = <&dma0 ...>, <&dma1 ...>;
  dma-names = "tx", "rx";

So "tx" is the name for the <&dma0 ...> element, and "rx" is the name
for the <&dma1 ...> element, all in a single "dmas" property.

By contrast, pinctrl properties look like this:

  pinctrl-0 = <&foo &bar ...>;
  pinctrl-1 = <&baz &blub ...>;
  pinctrl-names = "default", "sleep";

Here, "default" is the name for the entire pinctrl-0 property.
Similarly, "sleep" is the name of the pinctrl-1 property. It's a
strange situation where the node itself is kind of a container for an
array of pin control properties, which Zephyr's bindings language
can't really capture and has some special case handling in edtlib.

This is easiest to handle with ad-hoc code. Add special case macros
for pinctrls.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>