refactor: do_panic: modify expec string for panic.

Current TF-A code was using spsr_el3 to decide panic from lower or el3
and using that in error string, so tests were assuming its from el2,
the string gets modified to say its from lower el not rely in string el2.

Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
Change-Id: Ib466c559296fd97e04a421dc49495239e6b7fd5c
3 files changed