tools: gpio: add option to report wall-clock time to gpio-event-mon

Add support for selecting the realtime clock for events.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20201014231158.34117-4-warthog618@gmail.com


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