Add a comment and i2c method that describes the board layout.

Tested: firmware booted to userspace.
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Brandon Kim <brandonkim@google.com>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Message-id: 20210608193605.2611114-3-venture@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>