samples: sensor: ds18b20: add sample for this temperature driver

This commit adds a sample for the maxim ds18b20 1-wire temperatue
sensor in a single-drop bus configuration.
Includes overlay files for nucleo_g0b1re und nrf52840dk_nrf52840 with
the required open drain configuration.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>