stmf32f4: soc: fix buildman compilation error

fix the following compilation error reported by buidlman:

       arm:  +   stm32f429-discovery
+arch/arm/mach-stm32/stm32f4/soc.c: In function 'arch_cpu_init':
+arch/arm/mach-stm32/stm32f4/soc.c:30:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
+  for (int i = 0; i < ARRAY_SIZE(stm32_region_config); i++)
+  ^
+arch/arm/mach-stm32/stm32f4/soc.c:30:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
+make[3]: *** [arch/arm/mach-stm32/stm32f4/soc.o] Error 1
+make[2]: *** [arch/arm/mach-stm32/stm32f4] Error 2
+make[1]: *** [arch/arm/mach-stm32] Error 2
+make: *** [sub-make] Error 2

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Vikas Manocha <vikas.manocha@st.com>

arch/arm/mach-stm32/stm32f4/soc.c

@@ -21,13 +21,15 @@ int arch_cpu_init(void)
 		{ 0x00000000, REGION_0, XN_DIS, PRIV_RW_USR_RW,
 		STRONG_ORDER, REGION_4GB },
 	};
+	int i;
+
 	configure_clocks();
 	/*
 	 * Configure the memory protection unit (MPU) to allow full access to
 	 * the whole 4GB address space.
 	 */
 	disable_mpu();
-	for (int i = 0; i < ARRAY_SIZE(stm32_region_config); i++)
+	for (i = 0; i < ARRAY_SIZE(stm32_region_config); i++)
 		mpu_config(&stm32_region_config[i]);
 	enable_mpu();