iio: gyro: Add itg3200

This patch adds support for the InvenSense itg3200.
The itg3200 is a three-axis gyro with 16-bit ADC and
I2C interface.

Signed-off-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>