pico-imx7d: Convert DM MMC

This patch enable convert DM MMC for imx7d-pico board and variant.

Before the DM conversion only usdhc3 was enabled and therefore it appeared
as MMC 0 to u-boot. After enabling MMC DM though usdhc3 defaults to MMC 2,
which left unattended would drive changes to existing pico-pi bootscripts and
environment variables that rely on mmc 0.

Setup the alias of mmc0 and usdhc3 so that existing pico-imx7d boot code will
work unmodified.

When converting to DM_MMC it is necessary that SPL initializes eMMC
by itself, so move the original eMMC initialization from U-Boot
proper to SPL.

Signed-off-by: Joris Offouga <offougajoris@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>