feat(neoverse-rd): set interrupt priority for reboot interrupt

For reference design platforms, when the non-secure OS requests a
shutdown or reboot, one of the application cores will invoke a PSCI
call to enter TF-A. Upon receiving the PSCI command, this CPU core
will notify the other online CPU cores using SGI 15. Set the priority
for SGI 15 to the highest available value (zero).

Change-Id: I5eb0618bad24c675350315dfd7bee8475b829d5c
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>