fix(plat/st): improve DDR get size function

Avoid parsing device tree every time when returning
the DDR size.
A cache flush on this size is also added because TZC400 configuration
is applied at the end of BL2 after MMU and data cache being turned off.
Configuration needs to retrieve the DDR size to generate the correct
region. Access to the size fails because the value is still in the data
cache. Flushing the size is mandatory.

Change-Id: I3dd1958f37d806f9c15a5d4151968935f6fe642e
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>

plat/st/common/stm32mp_dt.c

@@ -209,15 +209,24 @@ int dt_get_stdout_uart_info(struct dt_node_info *info)
  ******************************************************************************/
 uint32_t dt_get_ddr_size(void)
 {
+	static uint32_t size;
 	int node;
 
+	if (size != 0U) {
+		return size;
+	}
+
 	node = fdt_node_offset_by_compatible(fdt, -1, DT_DDR_COMPAT);
 	if (node < 0) {
 		INFO("%s: Cannot read DDR node in DT\n", __func__);
 		return 0;
 	}
 
-	return fdt_read_uint32_default(fdt, node, "st,mem-size", 0);
+	size = fdt_read_uint32_default(fdt, node, "st,mem-size", 0U);
+
+	flush_dcache_range((uintptr_t)&size, sizeof(uint32_t));
+
+	return size;
 }
 
 /*******************************************************************************