arm/arm64: PL031: Fix check_rtc_irq

Since QEMU commit 83ad95957c7e ("pl031: Expose RTCICR as proper WC
register") the PL031 test gets into an infinite loop. Now we must
write bit zero of RTCICR to clear the IRQ status. Before, writing
anything to RTCICR would work. As '1' is a member of 'anything'
writing it should work for old QEMU as well.

Cc: Alexander Graf <graf@amazon.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Alexander Graf <graf@amazon.com>