bus: fsl-mc: add the fsl_mc_get_endpoint function

Using the newly added fsl_mc_get_endpoint function a fsl-mc driver can
find its associated endpoint (another object at the other link of a MC
firmware link).

The API will be used in the following patch in order to discover the
connected DPMAC object of a DPNI.

Also, the fsl_mc_device_lookup function is made available to the entire
fsl-mc bus driver and not just for the dprc driver.

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