Add getter function for Timer and LPTimer

With this API it is possible to retrieve the timer from sketch and
start/stop it using mbed APIs.

Author: giulcioffi

cores/arduino/Arduino.h

@@ -100,6 +100,7 @@ extern analogin_config_t adcCurrentConfig;
 #endif
 
 #include "Serial.h"
+#include "timer.h"
 #if defined(SERIAL_CDC)
 #define Serial _UART_USB_
 #define SerialUSB _UART_USB_

cores/arduino/timer.h

@@ -0,0 +1,8 @@
+#include "mbed.h"
+
+enum TimerType {
+  TIMER = 0x1,
+  LPTIMER = 0x2
+};
+
+mbed::Timer* getTimer(TimerType t = TIMER);

cores/arduino/wiring.cpp

@@ -68,6 +68,15 @@ void init()
   lowPowerTimer.start();
 }
 
+mbed::Timer* getTimer(TimerType t)
+{
+  if (t == LPTIMER) {
+    return (mbed::Timer*)(&lowPowerTimer);
+  } else {
+    return &timer;
+  }
+}
+
 void yield() {
 #ifndef NO_RTOS
   rtos::ThisThread::yield();