staging: fsl-mc: add function to return pointer to root dprc

To support multiple root dprcs, instead of relying on the
dev_root field of the bus type struct, instead create a
function to traverse to the root dprc and return a pointer
to the device struct

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