doc: imx: psb: Add documentation for MX8MM behavior with Fast Boot fuse blown

doc: imx: psb: Add documentation for MX8MM behavior with Fast Boot fuse blown

On iMX8MM with Fast Boot fuse blown, the SIT and A-copy image are
placed at different offset than on iMX8MM with Fast Boot fuse NOT
blown. List both options and both offsets to avoid confusion.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>