ARM: arm: colibri-imx6ull-emmc: fix emmc access

Synchronizing the device tree with linux introduced a regression.
The U-Boot specific dtsi mustn't override the alias settings for
the eMMC/SD interfaces.

Without this U-Boot cannot access the eMMC and boot the kernel.

Fixes: c21b61bf ("colibri-imx6ull/-emmc: synchronise device tree with linux")
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>