Add an example SPI slave handler responding with the uptime at the time
of reception of the last SPI message.

This can be used by an external microcontroller as a dead man's switch.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NMark Brown <broonie@kernel.org>