Documentation: dpaa2: Use correct heading adornment

Add overline heading adornment to document title in order to comply
with kernel doc requirements.

Fixes: 60b91319 staging: fsl-mc: Convert documentation to rst format

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>