staging: fsl-mc: Move DPCON out of staging

Move the source files out of staging into their final locations:
- dpcon.c goes to drivers/bus/fsl-mc/, next to the core infrastructure
- dpcon-cmd.h gets merged into drivers/bus/fsl-mc/fsl-mc-private.h, next
  to the other internally used APIs
- dpcon.h gets merged into include/linux/fsl/mc.h, exposing the public
  API

Update references in the dpaa2-eth staging driver.

DPCON stands for Data Path Concentrator - an interface between DPIO
(Data Path IO) and its users (e.g. dpaa2-eth). You can read more about
DPIO in Documentation/networking/dpaa2/overview.rst

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Reviewed-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>