Add api to free I2C object

ARMmbed#15191

Author: goshawk22

drivers/include/drivers/I2C.h

@@ -168,6 +168,10 @@ class I2C : private NonCopyable<I2C> {
      */
     int write(int data);
 
+    /** Frees the i2c object, allowing us to sleeps
+     */
+    void free(void);
+
     /** Creates a start condition on the I2C bus
      */
     void start(void);

drivers/source/I2C.cpp

@@ -113,6 +113,13 @@ int I2C::read(int ack)
     return ret;
 }
 
+void I2C::free(void) 
+{
+    lock();
+    i2c_free(&_i2c);
+    unlock();
+}
+
 void I2C::start(void)
 {
     lock();