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 d0399a46e7 ("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: d0399a46e7 ("imx6dl/imx6qdl: synchronise device trees with linux")
Signed-off-by: Fabio Estevam <festevam@denx.de>

arch/arm/dts/imx6dl-pico-u-boot.dtsi

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+
+/ {
+	aliases {
+		mmc0 = &usdhc3;
+	};
+};