irqchip/gic-v2: Reset APRn registers at boot time

Booting a crash kernel while in an interrupt handler is likely
to leave the Active Priority Registers with some state that
is not relevant to the new kernel, and is likely to lead
to erratic behaviours such as interrupts not firing as their
priority is already active.

As a sanity measure, wipe the APRs clean on startup.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>