doc: fix build with sphinx 5.0 and later.

Sphinx 5.0 and later fails to build when language is set to None:
Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English)

Let's set the language to English since it is the language used for the
documentation.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>