plat/st: do not rely on tainted value for dt property length

To compare the "okay" string of a property, strncmp is used but with the
length given by fdt_getprop. This len value is reported as tainted by
Coverity [1]. We just can use strlen("okay") which is a known value
to compare the 2 strings.

 [1] https://scan4.coverity.com/reports.htm#v51972/p11439/fileInstanceId=96515154&defectInstanceId=14219121&mergedDefectId=342997

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: Ic8fb6ccf3126a37df615e433eb028861812015da

plat/st/common/stm32mp_dt.c

 /*
- * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2021, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -72,21 +72,20 @@ bool fdt_check_node(int node)
 uint8_t fdt_get_status(int node)
 {
 	uint8_t status = DT_DISABLED;
-	int len;
 	const char *cchar;
 
-	cchar = fdt_getprop(fdt, node, "status", &len);
+	cchar = fdt_getprop(fdt, node, "status", NULL);
 	if ((cchar == NULL) ||
-	    (strncmp(cchar, "okay", (size_t)len) == 0)) {
+	    (strncmp(cchar, "okay", strlen("okay")) == 0)) {
 		status |= DT_NON_SECURE;
 	}
 
-	cchar = fdt_getprop(fdt, node, "secure-status", &len);
+	cchar = fdt_getprop(fdt, node, "secure-status", NULL);
 	if (cchar == NULL) {
 		if (status == DT_NON_SECURE) {
 			status |= DT_SECURE;
 		}
-	} else if (strncmp(cchar, "okay", (size_t)len) == 0) {
+	} else if (strncmp(cchar, "okay", strlen("okay")) == 0) {
 		status |= DT_SECURE;
 	}