[POWERPC] cell: Fix undefined reference to mmio_nvram_init

This fixes the following link error with CONFIG_PPC_CELL_NATIVE=y and
CONFIG_PPC_CELL_BLADE=n:

arch/powerpc/platforms/built-in.o: In function `.cell_setup_arch':
setup.c:(.init.text+0xe80): undefined reference to `.mmio_nvram_init'
Signed-off-by: Kou Ishizaki <Kou.Ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/platforms/cell/setup.c

@@ -179,7 +179,9 @@ static void __init cell_setup_arch(void)
 	conswitchp = &dummy_con;
 #endif
 
+#ifdef CONFIG_MMIO_NVRAM
 	mmio_nvram_init();
+#endif
 }
 
 static int __init cell_probe(void)