docs: networking: dpaa2: Fix DPNI header

The DPNI object should get its own header, like the rest of the objects.

Fixes: 60b91319 ("staging: fsl-mc: Convert documentation to rst format")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>