ARM64: dts: meson-gxbb: add the SDIO pins

This is used to configure the pins of the sd_emmc_a controller to
which an SDIO module is connected (when available).

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>