Documentation: add cleanmediadocs to the documentation targets

This was broken when updating the documentation targets for the Sphinx
build, and moving from %docs target pattern to explicitly listed
targets.

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Fixes: 22cba31bae9d ("Documentation/sphinx: add basic working Sphinx configuration and build")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile.sphinx

@@ -56,6 +56,7 @@ sgmldocs:
 psdocs:
 mandocs:
 installmandocs:
+cleanmediadocs:
 
 cleandocs:
 	$(Q)rm -rf $(BUILDDIR)

Makefile

@@ -1414,7 +1414,7 @@ $(help-board-dirs): help-%:
 
 # Documentation targets
 # ---------------------------------------------------------------------------
-DOC_TARGETS := xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs
+DOC_TARGETS := xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs cleanmediadocs
 PHONY += $(DOC_TARGETS)
 $(DOC_TARGETS): scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts build_docproc build_check-lc_ctype