iio:bma180: Implement _available sysfs attribute dynamically

makes it easier to add more chip variants and removes redundancy:
scales and frequencies are now stated just once

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>