arm64: timer: Write to ICENABLER to disable timer IRQ

According the Generic Interrupt Controller versions 2, 3 and 4 architecture
specifications, a write of 0 to the GIC{D,R}_ISENABLER{,0} registers is
ignored; this is also how KVM emulates the corresponding register. Write
instead to the ICENABLER register when disabling the timer interrupt.

Note that fortunately for us, the timer test was still working as intended
because KVM does the sensible thing and all interrupts are disabled by
default when creating a VM.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>