drivers: sensor: zephyr_thermistor: align connection type with Linux

It looks like the Zephyr thermistor driver bindings were half-copied
from Linux ntc-thermistor. Zephyr principle is to maintain compatibility
with Linux, when possible, so there's no reason to deviate here. Convert
the connection type from a custom enum to a boolean, as Linux does.

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