pinctrl: mvebu: armada-39x: normalize SDIO pin naming

In order to be consistent with the datasheet and some other SoCs, this
commit renames the SDIO pins of the Armada 39x from "sd" to "sd0".

While this changes the DT binding, this is not a problem since Armada
39x is a brand new SoC which isn't used in production yet (so now is
the right time to fix such things).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>