doc: shorten overlong title underlines

Title underlines should match the length of the title. Unfortunately
docutils only catches underlines that are too short.

Add some missing empty lines after titles.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/board/st/st-dt.rst b/doc/board/st/st-dt.rst
index 67e16ef..2a285c8 100644
--- a/doc/board/st/st-dt.rst
+++ b/doc/board/st/st-dt.rst
@@ -2,7 +2,7 @@
 .. sectionauthor:: Patrick Delaunay <patrick.delaunay@foss.st.com>
 
 U-Boot device tree bindings
-----------------------------
+---------------------------
 
 The U-Boot specific bindings are defined in the U-Boot directory:
 doc/device-tree-bindings
diff --git a/doc/board/st/stm32_MCU.rst b/doc/board/st/stm32_MCU.rst
index 7ff7c73..61650bc 100644
--- a/doc/board/st/stm32_MCU.rst
+++ b/doc/board/st/stm32_MCU.rst
@@ -2,7 +2,7 @@
 .. sectionauthor:: Patrice Chotard <patrice.chotardy@foss.st.com>
 
 STM32 MCU boards
-=================
+================
 
 This is a quick instruction for setup STM32 MCU boards.