arm: bootm-fdt.c: fix compiler warning

compiling U-Boot with bosch_mpcxxxxd_sd_defconfig
drops warning:

arch/arm/lib/bootm-fdt.c: In function ‘arch_fixup_fdt’:
arch/arm/lib/bootm-fdt.c:37:6: warning: unused variable ‘ret’ [-Wunused-variable]
  int ret = 0;
      ^~~

Fix it.
Signed-off-by: NHeiko Schocher <hs@denx.de>

arch/arm/lib/bootm-fdt.c

@@ -34,7 +34,7 @@ __weak int fdt_update_ethernet_dt(void *blob)
 
 int arch_fixup_fdt(void *blob)
 {
-	int ret = 0;
+	__maybe_unused int ret = 0;
 #if defined(CONFIG_ARMV7_NONSEC) || defined(CONFIG_OF_LIBFDT)
 	bd_t *bd = gd->bd;
 	int bank;