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 <festevam@denx.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>