staging: fsl-mc: add counter to track number of root DPRCs

Add a counter to track the number of root DPRCs.
When this counter is greater then 0 it means that at least
one root DPRC device exists.

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