iio: gyro: itg3200: add suspend/resume support.

Unless we put the device to sleep when not it use, it wastes
6mA.

If the device is asleep on probe, the 'id' register
sometimes mis-reads - so reset first.  If the device responds
at all a command sent to the address, it is almost certainly
the correct device already.

Acked-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>