ARM: defconfig: Set CONFIG_MMC_TEST as module

In some rare case, it is observed that mmc driver is
getting registered as mmc_test instead of mmcblk and
resulting in NULL pointer deference issue for sdcard.
To fix this, set CONFIG_MMC_TEST as module to make sure
mmcblk will get registered first.

Change-Id: Ia0f31a622ba88ee81ad7061dde2a392ba30a6be6
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>