Add DM_MMC to all defconfigs whose board is
nitrogen6x.

mx6qsabrelite_defconfig
nitrogen6dl2g_defconfig
nitrogen6dl_defconfig
nitrogen6q2g_defconfig
nitrogen6q_defconfig
nitrogen6s1g_defconfig

This commit is adding multiple configs, but
seems to be the minimum change to not lose features
and still compile fine.
Signed-off-by: NTroy Kisky <troy.kisky@boundarydevices.com>