ARM: imx_v6_v7_defconfig: Let CONFIG_SCSI_LOWLEVEL be selected

When using Yocto's Ptest DISTRO_FEATURE the CONFIG_SCSI_DEBUG=m option
is added, but it cannot be selected as it depends on CONFIG_SCSI_LOWLEVEL.

This generates a build warning saying that the CONFIG_SCSI_DEBUG=m option
is discarded.

Fix this by letting CONFIG_SCSI_LOWLEVEL to be selected.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>