ARM: imx: Use correct U-Boot offset in case of secondary boot from eMMC

In case of a secondary image boot from the user area of an eMMC device,
the correct offset must be calculated. The offset is fused in the fuse
IMG_CNTN_SET1_OFFSET of the i.MX8M Nano and Plus. The calculation of the
offset is described in the reference manual (IMX8MNRM Rev. 2, 07/2022
and IMX8MPRM Rev. 1, 06/2021):

The fuse IMG_CNTN_SET1_OFFSET (0x490[22:19]) is defined as follows:
* Secondary boot is disabled if fuse value is bigger than 10,
  n = fuse value bigger than 10.
* n == 0: Offset = 4MB
* n == 2: Offset = 1MB
* Others & n <= 10 : Offset = 1MB*2^n

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