[radio] use stack reset in RCP (#6645)

This commit makes RCP use a stack reset instead of doing a hard reset
when a reset command is received.

tests/unit/test_platform.cpp

@@ -550,4 +550,11 @@ bool otPlatCryptoHasKey(otCryptoKeyRef aKeyRef)
 
 #endif // OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE
 
+otError otPlatRadioSetCcaEnergyDetectThreshold(otInstance *aInstance, int8_t aThreshold)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+    OT_UNUSED_VARIABLE(aThreshold);
+
+    return OT_ERROR_NONE;
+}
 } // extern "C"