staging: fsl-mc: dpmcp: drop unused APIs

These APIs are not used yet, so drop the dead code.
The patch is mostly mechanical, with a couple exceptions:
 - getters/setters were not removed even if only one of
   them is being used
 - versioning API was also left in place
Also in this patch, add missing prototype for
version query function.

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