drivers: stm32_rng: use default PRNG init if RNG is disabled in the DT

In case RNG peripheral is not enabled in device tree, fallback to the
default RNG initialization implementation.

Signed-off-by: Thomas Bourgoin <thomas.bourgoin@foss.st.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>

core/drivers/stm32_rng.c

@@ -428,11 +428,17 @@ static TEE_Result stm32_rng_read(uint8_t *out, size_t size)
 }
 
 #ifdef CFG_WITH_SOFTWARE_PRNG
-/* Override weak plat_rng_init with platform handler to seed PRNG */
+/* Override weak plat_rng_init with platform handler to attempt to seed PRNG */
 void plat_rng_init(void)
 {
 	uint8_t seed[RNG_FIFO_BYTE_DEPTH] = { };
 
+	if (!stm32_rng) {
+		__plat_rng_init();
+		DMSG("PRNG seeded without RNG");
+		return;
+	}
+
 	if (stm32_rng_read(seed, sizeof(seed)))
 		panic();