MIPS: ath79: fix build problem if CONFIG_BLK_DEV_INITRD is not set

initrd_start is defined in init/do_mounts_initrd.c, which is only
included in kernel if CONFIG_BLK_DEV_INITRD=y.
Signed-off-by: Laurent Fasnacht <l@libres.ch>
Cc: linux-mips@linux-mips.org
Cc: trivial@kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10198/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ath79/prom.c

 /*
  *  Atheros AR71XX/AR724X/AR913X specific prom routines
  *
+ *  Copyright (C) 2015 Laurent Fasnacht <l@libres.ch>
  *  Copyright (C) 2008-2010 Gabor Juhos <juhosg@openwrt.org>
  *  Copyright (C) 2008 Imre Kaloz <kaloz@openwrt.org>
  *
@@ -25,12 +26,14 @@ void __init prom_init(void)
 {
 	fw_init_cmdline();
 
+#ifdef CONFIG_BLK_DEV_INITRD
 	/* Read the initrd address from the firmware environment */
 	initrd_start = fw_getenvl("initrd_start");
 	if (initrd_start) {
 		initrd_start = KSEG0ADDR(initrd_start);
 		initrd_end = initrd_start + fw_getenvl("initrd_size");
 	}
+#endif
 }
 
 void __init prom_free_prom_memory(void)