Revert "imx: rom api: fix image offset computation"

This reverts commit 1f63ee65.

As reported by Ye Li on ML:

1. Removing the image_offset will break secondary (redundant) boot support for sd and emmc.
2. When booting from emmc boot partition, the image_offset is 0. But the flash.bin
generated by mkimage with imximage-8mp-lpddr4.cfg is for sd. It expects to be burn at 32KB offset.
The fit offset 0x60000 has already included the 32KB offset.  So when you burn this flash.bin
to emmc boot partition at offset 0, the fit offset should subtract the 32KB (0x60000 - 0x8000).

Signed-off-by: Stefano Babic <sbabic@denx.de>