pico-imx6ul: Fix eMMC boot after DM_MMC conversion

After the DM_MMC conversion the following eMMC boot error is observed:

U-Boot SPL 2019.04-rc4 (Mar 20 2019 - 18:53:28 +0000)
Trying to boot from MMC1
MMC Device 0 not found
spl: could not find mmc device 0. error: -19
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

This happens because the SPL code does not initialize the SDHC pins
and clock.

Fix it by moving the original eMMC initialization from U-Boot proper
to SPL.

Reported-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Fabio Berton <fabio.berton@ossystems.com.br>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>