i2c: add api call to read byte from a specified register

Signed-off-by: Anton Krasovsky <anton@no-mad.net>
Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

api/mraa/i2c.hpp

@@ -143,6 +143,18 @@ class I2c {
             return mraa_i2c_write(m_i2c, buf, 2);
         }
 
+        /**
+         * Read an i2c register
+         *
+         * @param reg Register to read from
+
+         * @return char read from the bus
+         * @return char read from register
+         */
+        int readReg(char reg) {
+            return mraa_i2c_read_byte_data(m_i2c, reg);
+        }
+
         /**
          * Write multiple bytes to the bus
          *