dt-bindings: pinctrl: mcp23s08: add documentation for reset-gpios

The reset-gpios property is added to the optional dt-bindings and
also an example for it's usage.

Signed-off-by: Andreas Kaessens <akaessens@gmail.com>
Signed-off-by: Darian Biastoch <d.biastoch@gmail.com>
Link: https://lore.kernel.org/r/20210610132438.3085841-2-akaessens@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>