ARM: multi_v7_defconfig: enable the Meson MX SDIO/MMC controller

This enables the new Amlogic Meson MX SDIO/MMC controller driver, which
typically connects to the SD card and SDIO wifi module.
The driver is enabled as built-in to allow booting from the SD card.

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