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

commit 948588e25b8af5e66962ed3f53e1cae1656fa5af upstream.

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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>