ARM: imx: Factor out parsing of ROM log

Factor out parsing of ROM log in function spl_mmc_emmc_boot_partition().
This can be helpful to detect a secondary image boot without fiddling
around with MMC partitions. This way for example, U-Boot is able to
detect a secondary image boot and can enter some fallback scenario like
starting a recovery mode.

Signed-off-by: Fedor Ross <fedor.ross@ifm.com>
Signed-off-by: Marek Vasut <marex@denx.de>