staging: fsl-mc: remove references to dev_root

The dev_root field in the bus type struct has been replaced by a
new mechanism to identify the root dprc.  Remove all references
to dev_root.

Signed-off-by: Itai Katz <itai.katz@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>