spi: slave: Add SPI slave handler reporting uptime at previous message

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: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>