dt-bindings: i2c: hisilicon,hix5hd2: convert to DT schema

Convert the Devicetree binding documentation for hisilicon,hix5hd2-i2c
from plain text to DT binding schema.

Signed-off-by: Kael D'Alcamo <dev@kael-k.io>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>