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/ti/am62x_sk.rst b/doc/board/ti/am62x_sk.rst
index d7437c6..4703ce6 100644
--- a/doc/board/ti/am62x_sk.rst
+++ b/doc/board/ti/am62x_sk.rst
@@ -2,7 +2,7 @@
 .. sectionauthor:: Vignesh Raghavendra <vigneshr@ti.com>
 
 AM62 Platforms
-===============
+==============
 
 Introduction:
 -------------
@@ -117,7 +117,8 @@
 .. am62x_evm_rst_include_end_build_steps
 
 Target Images
---------------
+-------------
+
 In order to boot we need tiboot3.bin, tispl.bin and u-boot.img.  Each SoC
 variant (GP, HS-FS, HS-SE) requires a different source for these files.