aio: add calls for changing bit value of the read

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

include/mraa_internal_types.h

@@ -76,4 +76,5 @@ struct _pwm {
 struct _aio {
     unsigned int channel; /**< the channel as on board and ADC module */
     int adc_in_fp; /**< File Pointer to raw sysfs */
+    int value_bit; /**< 10 bits by default. Can be increased if board */
 };