mmc: jz4740: Let the pinctrl driver configure the pins

Now that the JZ4740 and similar SoCs have a pinctrl driver, we rely on
the pins being properly configured before the driver probes.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>