powerpc/mpic: create mpic subsystem object

Register a mpic subsystem at /sys/devices/system/

Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
diff --git a/arch/powerpc/include/asm/mpic.h b/arch/powerpc/include/asm/mpic.h
index ea6bf72..4a1ac9f 100644
--- a/arch/powerpc/include/asm/mpic.h
+++ b/arch/powerpc/include/asm/mpic.h
@@ -339,6 +339,8 @@
 #endif
 };
 
+extern struct bus_type mpic_subsys;
+
 /*
  * MPIC flags (passed to mpic_alloc)
  *