Enabling PWM on pins A1/A2.

Pins A1/A2 uses TC4 to generate PWM, the same used in Servo library,
so both cannot work together.

variants/arduino_zero/variant.cpp

@@ -139,8 +139,8 @@ const PinDescription g_APinDescription[]=
   // 14..19 - Analog pins
   // --------------------
   { PORTA,  2, PIO_ANALOG, PIN_ATTR_ANALOG, ADC_Channel0, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_2 }, // ADC/AIN[0]
-  { PORTB,  8, PIO_ANALOG, 0, ADC_Channel2, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_8 }, // ADC/AIN[2]
-  { PORTB,  9, PIO_ANALOG, 0, ADC_Channel3, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_9 }, // ADC/AIN[3]
+  { PORTB,  8, PIO_ANALOG, (PIN_ATTR_PWM|PIN_ATTR_TIMER), ADC_Channel2, PWM4_CH0, TC4_CH0, EXTERNAL_INT_8 }, // ADC/AIN[2]
+  { PORTB,  9, PIO_ANALOG, (PIN_ATTR_PWM|PIN_ATTR_TIMER), ADC_Channel3, PWM4_CH1, TC4_CH1, EXTERNAL_INT_9 }, // ADC/AIN[3]
   { PORTA,  4, PIO_ANALOG, 0, ADC_Channel4, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_4 }, // ADC/AIN[4]
   { PORTA,  5, PIO_ANALOG, 0, ADC_Channel5, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_5 }, // ADC/AIN[5]
   { PORTB,  2, PIO_ANALOG, 0, ADC_Channel10, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_2 }, // ADC/AIN[10]