RFC: ARM64: dts: Add Low-Speed Connector to ZCU100

This adds the low-speed connector to the ZCU100 rev C device
tree (also known as the Ultra96 board).

This is a proof-of-concept only, showing how it is possible
to populate a Secure96 board using the other patches in the
series.

If you comment out or delete the board {} node, you can
populate/depopulate the board from sysfs instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>