dt-bindings: pinctrl: mt8192: Add wrapping node for pin configurations

On mt8192, the pinctrl node has pinctrl groups to group pin
configurations. Each pinctrl group contains one or more pinmux subnodes
to list needed pins and their configurations. By supporting multiple
subnodes, we can configure different pin characteristics
(driving/pull-up/pull-down/etc.) in a pinctrl group.

Update the mt8192 pinctrl dt-binding to add the missing pinctrl group
node that wraps the pinmux subnodes and update the example at the end.
While at it, also remove the example embedded in the description since
it is redundant to the already supplied example at the end.

This same change was done for mt8195 in commit 79dcd4e8

 ("dt-bindings:
pinctrl: mt8195: add wrapping node of pin configurations").

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220315211936.442708-2-nfraprado@collabora.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>