soc: fsl: dpio: cleanup the cpu array on dpaa2_io_down

The dpio_by_cpu array should not contain a reference to a freed dpaa2_io
object. This patch adds the necessary cleanup in dpaa2_io_down.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>