docs(changelog): add vendor subsections for FDTs

To ease sorting fdts patches in changelog, add platform vendor
subsections. There is no dedicated scope.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I4dea2e81cf3f7a970a8531d542d7fe15a99656e5
diff --git a/changelog.yaml b/changelog.yaml
index 93eeb73..029b927 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -1287,31 +1287,37 @@
           - fdt
 
         subsections:
-          - title: Morello
-            scope: morello-fdts
+          - title: Arm
 
-            deprecated:
-              - fdts/morello
+            subsections:
+              - title: Morello
+                scope: morello-fdts
 
-          - title: STM32MP1
-            scope: stm32mp1-fdts
+                deprecated:
+                  - fdts/morello
 
-            deprecated:
-              - fdts stm32mp1
+          - title: ST
 
             subsections:
-              - title: STM32MP13
-                scope: stm32mp13-fdts
+              - title: STM32MP1
+                scope: stm32mp1-fdts
 
-              - title: STM32MP15
-                scope: stm32mp15-fdts
+                deprecated:
+                - fdts stm32mp1
 
-          - title: STM32MP2
-            scope: stm32mp2-fdts
+                subsections:
+                  - title: STM32MP13
+                    scope: stm32mp13-fdts
 
-            subsections:
-              - title: STM32MP25
-                scope: stm32mp25-fdts
+                  - title: STM32MP15
+                    scope: stm32mp15-fdts
+
+              - title: STM32MP2
+                scope: stm32mp2-fdts
+
+                subsections:
+                  - title: STM32MP25
+                    scope: stm32mp25-fdts
 
       - title: PIE
         scope: pie