doc: Silence sub-make output in sphinx builds

Add -s to the sub-make invocation so that the "Nothing to be done"
message is not shown.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/Makefile

@@ -52,7 +52,7 @@ loop_cmd = $(echo-cmd) $(cmd_$(1)) || exit;
 #    e.g. "media" for the linux-tv book-set at ./doc/media
 
 quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
-      cmd_sphinx = $(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=doc/media $2 && \
+      cmd_sphinx = $(MAKE) -s BUILDDIR=$(abspath $(BUILDDIR)) $(build)=doc/media $2 && \
 	PYTHONDONTWRITEBYTECODE=1 \
 	BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
 	$(SPHINXBUILD) \