bus: fsl-mc: add fsl_mc_allocator cleanup function

The userspace support for fsl-mc requires a fsl_mc_allocator
cleanup function. Add the needed function.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>