doc: update Kernel documentation build system

Update the docomentation build system according to Linux v5.11-rc1.

With this patch we can build the HTML documentation using either of
Sphinx 2 and Sphinx 3.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
[Fix warning in b4860qds.rst about "Switch Settings"]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/doc/sphinx/requirements.txt b/doc/sphinx/requirements.txt
index 742be3e..5030d34 100644
--- a/doc/sphinx/requirements.txt
+++ b/doc/sphinx/requirements.txt
@@ -1,3 +1,4 @@
-docutils==0.12
-Sphinx==1.4.9
+docutils
+Sphinx==2.4.4
 sphinx_rtd_theme
+six