bus: fsl-mc: extend fsl_mc_get_endpoint() to pass interface ID

In case of a switch DPAA2 object, the interface ID is also needed when
querying for the object endpoint. Extend fsl_mc_get_endpoint() so that
users can also pass the interface ID that are interested in.

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