pico-imx6: Pass the mmc alias to fix boot regression

Originally, the mmc aliases node was present in imx6qdl-pico.dtsi.

After the sync with Linux in commit d0399a46 ("imx6dl/imx6qdl:
synchronise device trees with linux"), the aliases node is gone as
the upstream version does not have it.

This causes a boot regression in which the eMMC card cannot be found anymore.

Fix it by passing the alias node in the u-boot.dtsi file to
restore the original behaviour where the eMMC (esdhc3) was
mapped to mmc0.

Fixes: d0399a46 ("imx6dl/imx6qdl: synchronise device trees with linux")
Signed-off-by: Fabio Estevam <festevam@denx.de>