HMP: Restrict irq_default_affinity to hmp_slow_cpu_mask

HMP: Restrict irq_default_affinity to hmp_slow_cpu_mask

This patch limits the default affinity mask for all irqs to the cluster of
the little cpus.

This patch has the positive side effect that an irq thread which has its
IRQTF_RUNTHREAD set inside irq_thread() -> irq_wait_for_interrupt() will
not overwrite its struct task_struct->cpus_allowed with a full cpu mask of
desc->irq_data.affinity in irq_thread_check_affinity() essentially reverting
patch "HMP: experimental: Force all rt tasks to start on little domain."
for this irq thread.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>