doc: Describe the bootstd settings

The bootstd node provides some configuration properties. Add these to
the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
1 file changed