ARM: multi_v7_defconfig: add SDHCI for i.MX

Turn on SDHCI for i.MX support so machines can boot with local rootfs
on SD. Tested on a Wandboard Quad.

Signed-off-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>