ARM: config: Set CONFIG_MMC_TEST as module for sdm845

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
on sdm845 target. To fix this, set CONFIG_MMC_TEST as
module to make sure mmcblk will get registered first.

Change-Id: Ia9e9ac55ba1b4d83930bdb7edc3108b553eadeb0
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>