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 <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/firmware.c

@@ -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);