dt-bindings: gpio: fix trivial-gpio's schema id

dt-bindings: gpio: fix trivial-gpio's schema id

In case the trivial-gpio schema is referenced through a $ref like
/schemas/trivial-gpio.yaml to match its current schema ID, the following
error message is displayed:

Error in referenced schema matching $id: http://devicetree.org/schemas/trivial-gpio.yaml
Tried these paths (check schema $id if path is wrong):
/path/to/linux/Documentation/devicetree/bindings/trivial-gpio.yaml
/path/to/dtchema/schemas/trivial-gpio.yaml

Fix this by adding the 'gpio' folder to the schema's ID to match its
file path.

Fixes: f03a7f20b23c ("dt-bindings: gpio: Create a trivial GPIO schema")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250912165916.3098215-1-ioana.ciornei@nxp.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>