plat-stm32mp1: fix use after free in PMIC driver

Fix PMIC regulator levels arrays handling that missed a pointer reset
after the buffer is freed. At runtime, pmic_list_voltages() handler
function uses that reference and is expected to allocate back the
buffer in case non-secure world requests voltage enumeration for the
related regulator.

Fixes: a7990eb ("plat-stm32mp1: set voltage list at pmic driver init")
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>

core/arch/arm/plat-stm32mp1/drivers/stm32mp1_pmic.c

@@ -542,8 +542,10 @@ static TEE_Result release_voltage_lists(void)
 	for (n = 0; n < ARRAY_SIZE(pmic_regulators); n++) {
 		struct pmic_regulator_data *priv = pmic_regulators[n].priv;
 
-		if (priv && priv->voltages_level)
+		if (priv && priv->voltages_level) {
 			free(priv->voltages_level);
+			priv->voltages_level = NULL;
+		}
 	}
 
 	return TEE_SUCCESS;