imx: rom api: fix image offset computation

When not booting from FlexSPI, the offset computation is:

offset = image_offset + CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR * 512 - 0x8000;

When booting from SD card or eMMC user partition, image_offset is
0x8000. It is useless to add and remove 0x8000.
When booting from other device, image_offset is 0 so this computation is wrong.

Simplfy this computation to work on all booting devices.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>