docs: document do_panic() and panic() helper functions

panic() and do_panic() are widely used helper functions called when
encountering a critical failure that cannot be recovered from.
Document them in porting guide. Also, remove panic() documentation
from PSCI guide(where it is unused anyways).

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib0965cce56c03d0de5ac0d05d5714a6942793ede