staging: fsl-mc: remove dpmng API files

dpmng.h & dpmng.c files expose an API of just one function which is only
used by the bus driver. Move that single API in the bus source as static
and remove the two files.

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