staging: fsl-mc: make fsl_mc_get_root_dprc public

fsl_mc_get_root_dprc is needed by other components (e.g. vfio) to find
the root dprc

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