sensor: nrf5: temp: Allow alternative temp driver

This allows out-of-tree libraries to implement their own temperature
driver.

We allow selecting TEMP_NRF5 to aviod dependency loops

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>