tests: subsys: rtio: Set to 1cpu as it is not SMP-safe

The current api rtio_mpsc_pop is not SMP-safe. When muilti threads on
SMP are racing to pop the node, it will likely break the queue. Set
CONFIG_MP_MAX_NUM_CPUS to 1 to temporarily fix the issue.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>