powerpc: Make powerpc_firmware_features __read_mostly

We use firmware_has_feature quite a lot these days, so it's worth putting
powerpc_firmware_features into __read_mostly.

Signed-off-by: Anton Blanchard <redacted>
---

Index: powerpc.git/arch/powerpc/kernel/firmware.c
===================================================================

--- powerpc.git.orig/arch/powerpc/kernel/firmware.c	2010-02-05 14:57:48.579712760 +1100
+++ powerpc.git/arch/powerpc/kernel/firmware.c	2010-02-05 14:57:54.688461988 +1100

@@ -17,5 +17,5 @@
 
 #include <asm/firmware.h>
 
-unsigned long powerpc_firmware_features;
+unsigned long powerpc_firmware_features __read_mostly;
 EXPORT_SYMBOL_GPL(powerpc_firmware_features);