refactor(aarch64): remove weak links to el3_panic

Cleanup weak links to el3_panic and restrict crash_reporting usage
to bl31.

Crash reporting is not used with bl1, bl2 and weak linkage to el3_panic
is used, this can cause ambiguity in understanding the code so remove
this weak linkage and introduce funcs that should be used when we have
crash reporting for el3 panics.

Change-Id: Ic5c711143ba36898ef9574a078b8fa02effceb12
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>