refactor(bl31): use elx_panic for sysreg_handler64

When we reach sysreg_handler64 from any trap handling we are entering
this path from lower EL and thus we should be calling lower_el_panic
reporting mechanism to print panic report.

Make report_elx_panic available through assembly func elx_panic which
could be used for reporting any lower_el_panic.

Change-Id: Ieb260cf20ea327a59db84198b2c6a6bfc9ca9537
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>