soc nrf54lm20a: Fix entropy source

nordic,entropy-prng does not exist in Zephyr (it is part of NCS)
but we have in both the nordic,nrf-cracen-ctrdrbg which is
an actual source of true entropy and works with this SOC.
Let's use that instead.

Fixes failures to build targeting the nrf54lm20dk any test/sample
which uses the entropy driver.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

Author: aescolar

dts/arm/nordic/nrf54lm20a_enga_cpuapp.dtsi

@@ -17,7 +17,7 @@ nvic: &cpuapp_nvic {};
 
 / {
 	chosen {
-		zephyr,entropy = &prng;
+		zephyr,entropy = &rng;
 	};
 
 	soc {
@@ -31,9 +31,9 @@ nvic: &cpuapp_nvic {};
 		status = "disabled";
 	};
 
-	prng: prng {
-		compatible = "nordic,entropy-prng";
+	rng: rng {
 		status = "okay";
+		compatible = "nordic,nrf-cracen-ctrdrbg";
 	};
 };