irqchip: irq-xtensa-mx: fix initial IRQ affinity

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>