rtio: workq: Default workqueue to two threads with SPI/I2C/I3C

As to facilitate work items preemption, common for sensor read ops.

e.g:
- Sensor read (submitted through workqueue).
- SPI read (submitted through workqueue using default handler).

This requires at least 2 thread pools to allow SPI read to complete,
to then come back and complete sensor reading.

Signed-off-by: Luis Ubieda <luisf@croxel.com>