Like "audio-routing" drop the minItems: 2 from the "audio-widgets", because
any limit here - lower or upper- is rather meaningless.

This will also fix `dtbs_check` warnings like:
sound: audio-widgets: ['Speaker', '7J4-14 LEFT', 'Speaker', '7J4-11 RIGHT'] is too long

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>