docs(changelog): add subsections for STM32MP2

Now that STM32MP2 family has different types of SoCs, add relative
subsections for STM32MP2. The same is done for FDTS scopes.

Change-Id: I374215c44ce9515ca33b5882058a71db4886b80a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
diff --git a/changelog.yaml b/changelog.yaml
index e5d43d9..14121e2 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -621,6 +621,12 @@
             scope: stm32mp2
 
             subsections:
+              - title: STM32MP21
+                scope: stm32mp21
+
+              - title: STM32MP23
+                scope: stm32mp23
+
               - title: STM32MP25
                 scope: stm32mp25
 
@@ -1352,6 +1358,12 @@
                 scope: stm32mp2-fdts
 
                 subsections:
+                  - title: STM32MP21
+                    scope: stm32mp21-fdts
+
+                  - title: STM32MP23
+                    scope: stm32mp23-fdts
+
                   - title: STM32MP25
                     scope: stm32mp25-fdts