ARMv8/sec-firmware: fix a compile error

When enabled sec firmware framework, but lack of definition of
the marco SEC_FIRMWARE_FIT_IMAGE, SEC_FIRMEWARE_FIT_CNF_NAME
and SEC_FIRMWARE_TARGET_EL, there will be some build errors,
so give a default definition.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>