ARM: dts: imx6qdl-sabre: Add mmc aliases

If not specified, the mmc0 and mmc1 devices will be the devices
mmc@2190000 and mmc@2194000, which are in disabled state on the iMX.6
Sabrelite devices.

The actual SD card reader devices are the ones at mmc@2198000 and
mmc@219c000.

Set aliases to use the correct mmc devices order.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>