Makefile: enable parallel build

>From sphinx-build man page:

-j N     build in parallel with N processes where possible
         (special value "auto" will set N to cpu-count)

(From yocto-docs rev: 1847aeea8b88c84f821610803264955e51e31d2f)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    ?=
+SPHINXOPTS    ?= -j auto
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = .
 BUILDDIR      = _build