pinctrl: meson-gxbb: add the pins for the SDIO/sd_emmc_a controller

sd_emmc_a is used a controller for the SDIO modules. This adds the pin
configuration for the SDIO controller.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>