imx6: isiotmx6ul: Update SPL board boot order for eMMC

imx6: isiotmx6ul: Update SPL board boot order for eMMC

SPL mmc device index is get based on the boot device, like
- BOOT_DEVICE_MMC1 for mmc device 0
- BOOT_DEVICE_MMC2 for mmc device 1

Currently BOOT_DEVICE_MMC1 is setting both SD/eSD and MMC/eMMC
boot devices in i.MX, So u-boot is loading from mmc device 0 even
"if the board booting from SD/eSD or MMC/eMMC"

So, this patch set BOOT_DEVICE_MMC2 for MMC/eMMC so for MMC/eMMC
the u-boot is loading from mmc device 1 and the board file need to
take care if the board have different mmc device order intialization.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>