mmc: fsl_esdhc_spl: pre-PBL: implement redundancy support

QorIQ pre-PBL BootROM scans first 24 SD card sectors (each with fixed 512
bytes length) for boot signature. Implement same redundancy behavior in
fsl_esdhc_spl driver to allow loading proper U-Boot when boot sector is not
the first one.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>