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/nxp/ls1046ardb.rst b/doc/board/nxp/ls1046ardb.rst
index 49b4842..8c0bc82 100644
--- a/doc/board/nxp/ls1046ardb.rst
+++ b/doc/board/nxp/ls1046ardb.rst
@@ -54,7 +54,7 @@
 - ARM JTAG support
 
 Memory map from core's view
-----------------------------
+---------------------------
 
 ================== ================== ================ =====
 Start Address      End Address        Description      Size