cmd_nandbcb: Support secondary boot address of imx8mn

cmd_nandbcb: Support secondary boot address of imx8mn

Add support of secondary boot address for imx8mn. The secondary
boot address is hardcoded in the fuse. The value is calculated
from there according to the following description:

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
- For FlexSPI boot, the valid values are: 0, 1, 2, 3, 4, 5, 6, and 7.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>