ARMv8/sec_firmware: Remove SEC_FIRMWARE_FIT_CNF_NAME

The config to use for FIT images can be better specified by enabling
CONFIG_MULTI_DTB_FIT and implementing board_fit_config_name_match.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>