gpio/reset: move gpio-{poweroff|restart} DT doc to proper place

I did only find them after a fuzzy search, so let them be where one
would expect them.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>