refactor(aarch64): rename do_panic and el3_panic

Current panic call invokes do_panic which calls el3_panic, but now panic
handles only panic from EL3 anid clear separation to use lower_el_panic()
which handles panic from lower ELs.

So now we can remove do_panic and just call el3_panic for all panics.

Change-Id: I739c69271b9fb15c1176050877a9b0c0394dc739
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>