iio: accel: bma400: Add separate channel for step counter

Added channel for step counter which can be enable or disable
through the sysfs interface.

Signed-off-by: Jagath Jog J <jagathjog1996@gmail.com>
Link: https://lore.kernel.org/r/20220505133021.22362-6-jagathjog1996@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>