ppc: Conditionally compile bat_rw.c

Only a few PPC boards actually use the common BAT manipulation
functions, so only compile it for them.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index 2065b6d..cec7666 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -30,7 +30,7 @@
 SOBJS-y	+= ticks.o
 SOBJS-y	+= reloc.o
 
-COBJS-y	+= bat_rw.o
+COBJS-$(CONFIG_BAT_RW) += bat_rw.o
 COBJS-y	+= board.o
 COBJS-y	+= bootm.o
 COBJS-$(CONFIG_BOOTCOUNT_LIMIT) += bootcount.o