iio: imu: adis: Add self_test_reg variable

This patch adds a dedicated self_test_reg variable. This is also a step
to let new drivers make use of `adis_initial_startup()`. Some devices
use MSG_CTRL reg to request a self_test command while others use the
GLOB_CMD register.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>