configs: ls1028a: Correct CONFIG_ENV_SECT_SIZE

LS1028A-RDB/QDS has Micron "mt35xu02g" nor flash of size 256M with 128K
sector size. Correct CONFIG_ENV_SECT_SIZE value to 128K.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>