armv8: ls1046aqds: Fix NAND offset for Fman ucode and env

Fix a bug of 'commit 8104deb2 ("armv8: layerscape: Adjust memory
mapping for Flash/SD card on LS1046A")' as NAND block size is 256KB
on LS1046AQDS.

Signed-off-by: Gong Qianyu <Qianyu.Gong@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>