sphinx: change default language to 'en'

sphinx: change default language to 'en'

Fixes the following Sphinx warning (treated as error) starting
with 5.0 release:

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).

Signed-off-by: Martin Liska <mliska@suse.cz>
Message-id: e91e51ee-48ac-437e-6467-98b56ee40042@suse.cz
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>