ARM: mxs_defconfig: Select CONFIG_MMC_UNSAFE_RESUME

PM subsystem treats mmc card as removed during suspend.

If MMC is used to store the root file system, it is better to tell the kernel
not to treat it as a removable media, so select CONFIG_MMC_UNSAFE_RESUME for
such purpose.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>