dt-bindings: gpio: Add bindings for gpio-virtio

This patch adds binding for virtio GPIO controller, it is based on
virtio-device bindings.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/acf7402ef4aabc0ad6295c32846f2bef1cd9b56a.1627362340.git.viresh.kumar@linaro.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Rob Herring <robh@kernel.org>