i2c.hpp: Add function to get i2c default bus

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/common.hpp

@@ -280,4 +280,16 @@ getSubPlatformIndex(int pin_or_bus_id)
 {
     return mraa_get_sub_platform_index(pin_or_bus_id);
 }
+
+/**
+ * Get default i2c bus, board must be initialised.
+ *
+ * @param optional subplatform identifier
+ * @return default i2c bus for paltform
+ */
+inline int
+getDefaultI2cBus(int platform_offset=MRAA_MAIN_PLATFORM_OFFSET)
+{
+    return mraa_get_default_i2c_bus(platform_offset);
+}
 }