boards: heltec_wifi_lora32_v2: enable trng0

trng0 refers to the entropy source in the ESP32, as defined in
esp32.dtsi. It enables the configuration item ENTROPY_ESP32_RNG and
therefore offers an entropy driver, making the use of
CONFIG_TEST_RANDOM_GENERATOR unnecessary.

This change helps running the esp32_wifi_station sample.

Signed-off-by: Michael Laß <bevan@bi-co.net>