dt-bindings: pinctrl: toshiba,visconti: Drop redundant functions type

Referenced pinmux-node.yaml schema already defines type for "functions"
so $ref is redundant.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: krzk

Documentation/devicetree/bindings/pinctrl/toshiba,visconti-pinctrl.yaml

@@ -42,7 +42,6 @@ patternProperties:
       function:
         description:
           Function to mux.
-        $ref: /schemas/types.yaml#/definitions/string
         enum: [i2c0, i2c1, i2c2, i2c3, i2c4, i2c5, i2c6, i2c7, i2c8,
                spi0, spi1, spi2, spi3, spi4, spi5, spi6,
                uart0, uart1, uart2, uart3, pwm, pcmif_out, pcmif_in]