iio: imu: inv_mpu6050: add accel lpf setting for chip >= MPU6500

Starting from MPU6500, accelerometer dlpf is set in a separate
register named ACCEL_CONFIG_2.
Add this new register in the map and set it for the corresponding
chips.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>