dt-bindings: i2c: Add bindings for i2c-virtio

This patch adds binding for virtio I2C device, it is based on
virtio-device bindings.

Acked-by: Wolfram Sang <wsa@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/33c317b95097ce491845c697db1e8285e3ec1d41.1627362340.git.viresh.kumar@linaro.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Rob Herring <robh@kernel.org>