fix(supervisor): boards(b_u585i_iot02a): fix external flash syntax

The syntax which was used is outdated. The commit mirrors the configs which will be delivered in zephyr v.4.4.
When that happens, this can be reduced to only container storage_partition node.

Signed-off-by: Krisztian Szilvasi <34309983+kr-t@users.noreply.github.com>

Author: kr-t

src/samples/supervisor/zephyr/boards/b_u585i_iot02a.overlay

@@ -115,16 +115,29 @@
     };
 };
 
-// 64MB external flash
-&mx25lm51245 { // -el "/usr/local/STMicroelectronics/STM32Cube/STM32CubeProgrammer/bin/ExternalLoader/MX25LM51245G_STM32U585I-IOT02A.stldr"
-    reg = <0x70000000 DT_SIZE_M(64)>;
-    partitions {
-        /delete-node/ partition;
-
-        /* Use the whole flash for the filesystem. */
-        storage_partition: partition@0 {
-            label = "user_data";
-            reg = <0x00000000 DT_SIZE_M(64)>;
+&mx25lm51245 {
+    reg = <0>;
+    size = <DT_SIZE_M(512)>; /* 512 Megabits */
+    #address-cells = <1>;
+    #size-cells = <1>;
+
+    ranges = <0x0 0x70000000 DT_SIZE_M(64)>; /* Ext Flash mem-mapped to 0x70000000 */
+
+    ext_flash: ext-flash@0 {
+        compatible = "soc-nv-flash";
+        reg = <0x0 DT_SIZE_M(64)>;
+        write-block-size = <1>;
+        erase-block-size = <DT_SIZE_K(4)>;
+
+        partitions {
+            compatible = "fixed-partitions";
+            #address-cells = <1>;
+            #size-cells = <1>;
+
+            /* Use the first partition from flash. */
+            storage_partition: partition@0 {
+                reg = <0x0 DT_SIZE_M(16)>;
+            };
         };
     };
 };