Update dtsi with new pinctrl compatible string

This commit updates bcm11351.dtsi with the new compatible string for
the same driver.

Signed-off-by: Sherman Yin <syin@broadcom.com>
Reviewed-by: Matt Porter <mporter@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>