irqchip/gic-v3: Don't try to reset AP0Rn

Clearing AP0Rn has created a number of regressions, due to systems
that have SCR_EL3.FIQ set. Even when addressing some obvious bugs,
GIC500 platforms seem to act bizarrely (we are supposed to have
5 bits of priority, but PMR seems to behave as if we had 6...).

Drop the AP0Rn reset for the time being, it is unlikely to have any
effect if kexec-ing.

Fixes: d6062a6d irqchip/gic-v3: Reset APgRn registers at boot time
Reported-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>