ARM: multi_v7_defconfig: Enable CONFIG_MMC_MESON_MX_SDHC

Enable CONFIG_MMC_MESON_MX_SDHC so that SDHC host controller
on Amlogic SoCs boards enable support for eMMC and MMC drivers.

Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20210715092233.1084-1-linux.amoon@gmail.com