meson.build: Move SPHINX_ARGS to top level meson.build file

We're going to want to use SPHINX_ARGS in both docs/meson.build
and tests/qapi-schema/meson.build. Move the definition up to the
top level file so it is available to both subdirectories.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-14-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>