ARM: dts: imx7d-colibri-emmc: Add usdhc aliases

Add mmc aliases to ensure a consistent mmc device naming across the
Toradex SoM family, with this commit mmc0 is the on-module eMMC
boot device and the not available mmc interfaces are removed.

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>