docs: Makefile: honor V=0 for docs building

Reduce the number of displayed mesages when building the
docs with V=0.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/478c114a2399b68a18de94ee5f98649304f3903b.1603796153.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: mchehab

Documentation/Makefile

@@ -21,6 +21,10 @@ BUILDDIR      = $(obj)/output
 PDFLATEX      = xelatex
 LATEXOPTS     = -interaction=batchmode
 
+ifeq ($(KBUILD_VERBOSE),0)
+SPHINXOPTS    += "-q"
+endif
+
 # User-friendly check for sphinx-build
 HAVE_SPHINX := $(shell if which $(SPHINXBUILD) >/dev/null 2>&1; then echo 1; else echo 0; fi)