ENGR00320355 dts: imx6: mmc index fixed by controller order

ENGR00320355 dts: imx6: mmc index fixed by controller order

Make the linux mmc index to be fixed according to controller order.
This can make user easily to identify which mmcX corresponding to which
controller and kernel be able find the rootfs in a card plugged in a
specific slot persistently.

This is a eventually solution for finding mmc block devices correctly
for different cards on multi slots.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
(cherry picked from commit f53ed08c6af46fc46a244cf3f5f0b639cd41bd16)