imx: Distinguish between MMC1 and MMC2 boot settings

Based on a patch from Rabeeh Khoury <rabeeh@solid-run.com>

Signed-off-by: Steve Langasek <vorlon@debian.org>
Always use MMCSD_MODE_ANY in i.MX SPL

This enables support for fall-through from FAT to RAW on all SPL-using
i.MX systems (currently, cubox-i and ventana).

Signed-off-by: Steve Langasek <vorlon@debian.org>