sandbox: dts: Add the real-time-clock test nodes back in

These were lost when the PMIC series was applied. Add them back so that the
tests pass again.

Reported-by: Joe Hershberger <joe.hershberger@gmail.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>