plat/st: correct static analysis tool warning

Correct the following sparse warnings:
plat/st/common/stm32mp_dt.c:103:5: warning:
 symbol 'fdt_get_node_parent_address_cells' was not declared.
 Should it be static?
plat/st/common/stm32mp_dt.c:123:5: warning:
 symbol 'fdt_get_node_parent_size_cells' was not declared.
 Should it be static?

As those 2 functions are only used by assert(), put them under
ENABLE_ASSERTIONS flag.

Change-Id: Iad721f12128df83a3de3f53e7920a9c1dce64c56
Signed-off-by: Yann Gautier <yann.gautier@st.com>

plat/st/common/stm32mp_dt.c

 /*
- * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -92,6 +92,7 @@ uint8_t fdt_get_status(int node)
 	return status;
 }
 
+#if ENABLE_ASSERTIONS
 /*******************************************************************************
  * This function returns the address cells from the node parent.
  * Returns:
@@ -100,7 +101,7 @@ uint8_t fdt_get_status(int node)
  * - a default value if undefined #address-cells property as per libfdt
  *   implementation.
  ******************************************************************************/
-int fdt_get_node_parent_address_cells(int node)
+static int fdt_get_node_parent_address_cells(int node)
 {
 	int parent;
 
@@ -120,7 +121,7 @@ int fdt_get_node_parent_address_cells(int node)
  * - a default value if undefined #size-cells property as per libfdt
  *   implementation.
  ******************************************************************************/
-int fdt_get_node_parent_size_cells(int node)
+static int fdt_get_node_parent_size_cells(int node)
 {
 	int parent;
 
@@ -131,6 +132,7 @@ int fdt_get_node_parent_size_cells(int node)
 
 	return fdt_size_cells(fdt, parent);
 }
+#endif
 
 /*******************************************************************************
  * This function reads a value of a node property (generic use of fdt