soc: fsl: dpio: store a backpointer to the device backing the dpaa2_io

Add a new field in the dpaa2_io structure to hold a backpointer to the
actual DPIO device.

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