Convert the Fujitsu MB86S7x GPIO binding to DT schema format.

The "socionext,synquacer-gpio" compatible was not documented, but is
compatible with "fujitsu,mb86s70-gpio" and is in use (in u-boot
Synquacer dts).
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>