Convert the Arm PL061 GPIO controller binding to json-schema format.

As I'm the author for all but the gpio-ranges line, make the schema dual
GPL/BSD license.

Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>