iio: Add ABI documentation for scaled voltage

This patch adds an entry in ABI documentation for in_voltage-voltage_scale.
It has at least one user driver, adis16220, in accel driver.

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
1 file changed