configs: ls2088ardb: Correct DEFAULT_DEVICE_TREE value

LS2088A-RDB has CONFIG_DEFAULT_DEVICE_TREE value correctly set as
"fsl-ls2088a-rdb-qspi" for QSPI secure/non-secure boot and TFA
non-secure boot mode.

Fix the value for TFA secure boot mode.

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