irqchip/xtensa-mx: Fix initial IRQ affinity in non-SMP setup

irqchip/xtensa-mx: Fix initial IRQ affinity in non-SMP setup

When irq-xtensa-mx chip is used in non-SMP configuration its
irq_set_affinity callback is not called leaving IRQ affinity set empty.
As a result IRQ delivery does not work in that configuration.
Initialize IRQ affinity of the xtensa MX interrupt distributor to CPU 0
for all external IRQ lines.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220426161912.1113784-1-jcmvbkbc@gmail.com