drivers: sensor: ntc_thermistor: add support for generic NTC

Add support for a generic NTC, `ntc-thermistor-generic`. In this case,
the compensation table is provided via devicetree. Note that DT property
is prefixed with `zephyr,`, because while hardware related, it is linked
to a particular software implementation.

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