fix rivafb build on ppc64

Hi Linus, Andrew !

rivafb is part of the g5 defconfig, but will cause a build error on ppc64 due to a missing #include, here is a fix, please apply:

===== drivers/video/riva/fbdev.c 1.51 vs edited =====

@@ -44,6 +44,10 @@
 #ifdef CONFIG_MTRR
 #include <asm/mtrr.h>
 #endif
+#ifdef CONFIG_PPC_OF
+#include <asm/prom.h>
+#include <asm/pci-bridge.h>
+#endif
 
 #include "rivafb.h"
 #include "nvreg.h"