Fix markup warning: insert a blank line before the list.

Documentation/driver-api/gpio/board.rst:209: WARNING: Unexpected indentation.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>