sys: linear_range: initial API

This patch adds a new API to map values in a linear range to a range
index. This API is useful in the context of regulators, where it is
common that devices map regulator voltages to a set of indices with a
linear relationship.

The original idea is borrowed from Linux.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>