DISCO_F429ZI : add ADC_3 define to ARCH_MAX

Since DISCO_F429ZI uses 2 ADCs(#1 and #3), analogin_api was modified to
support it.
ARCH_MAX is based on STM32F407 product, that has 3 ADCs. ADC_3 thus
needs to be defined in the TARGET_ARCH_MAX/PeripharalNames.h (even if
not used)

libraries/mbed/targets/hal/TARGET_STM/TARGET_STM32F4/TARGET_ARCH_MAX/PeripheralNames.h

@@ -37,7 +37,8 @@ extern "C" {
 #endif
 
 typedef enum {
-    ADC_1 = (int)ADC1_BASE
+    ADC_1 = (int)ADC1_BASE,
+    ADC_3 = (int)ADC3_BASE
 } ADCName;
 
 typedef enum {