imx: Fix regression with CONFIG_DM_MMC=y

When CONFIG_DM_MMC=y, CONFIG_BLK should be selected, otherwise the
SD/eMMC card cannot be used.

Also, select CONFIG_DM_USB=y when CONFIG_USB=y to avoid build failure.

Tested on mx6slevk, mx7dsabresd and mx6ullevk.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Adam Ford <aford173@gmail.com>
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Tested-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>