sensor: nrf5: Convert name to come from device tree

Update the binding for nordic,nrf-temp to require the label property and
use the generated define (DT_INST_0_NORDIC_NRF_TEMP_LABEL) instead of
Kconfig symbol (CONFIG_TEMP_NRF5_NAME).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>