dt-bindings: gpio: fsl-imx-gpio: Document imx25 and imx27

fsl,imx25-gpio and fsl,imx27-gpio are not documented, causing schema
warnings.

fsl,imx25-gpio is compatible with fsl,imx35-gpio and fsl,imx27-gpio
is compatible with fsl,imx21-gpio.

Document them accordingly.

Signed-off-by: Fabio Estevam <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/devicetree/bindings/gpio/fsl-imx-gpio.yaml

···
           - fsl,imx35-gpio
           - fsl,imx7d-gpio
       - items:
+          - enum:
+              - fsl,imx27-gpio
+          - const: fsl,imx21-gpio
+      - items:
           - const: fsl,imx35-gpio
           - const: fsl,imx31-gpio
+      - items:
+          - enum:
+              - fsl,imx25-gpio
+          - const: fsl,imx35-gpio
       - items:
           - enum:
               - fsl,imx50-gpio