dt-bindings: pinctrl: mt8195: add wrapping node of pin configurations

On mt8195, the pinctrl node has pinctrl groups to group pin
configurations by users' need. In each pinctrl group, it has
subnode(s) to list pins needed and pin configurations. By supporting
multiple subnodes, we can configure different pin characteristics
(driving/pull-up/pull-down/etc.) in a pinctrl group.

Update pinctrl-mt8195.yaml to add subnode in pinctrl groups and an
example to illustrate the usage.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Acked-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211220121825.6446-4-tinghan.shen@mediatek.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>