dt-bindings: pinctrl: Fix node descriptions in uniphier-pinctrl example

Drop parent node of the pinctrl as it is not directly necessary, and
add more examples, that is "groups", "function", and a child node to set
pin attributes, to express this pinctrl node in detail.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20221213082449.2721-4-hayashi.kunihiko@socionext.com


Signed-off-by: Rob Herring <robh@kernel.org>