dt-bindings: Add binding for IT6505.

Add a DT binding documentation for IT6505.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Allen Chen <allen.chen@ite.com.tw>
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
[fixed example to use i2c]
Link: https://patchwork.freedesktop.org/patch/msgid/1587979103-5630-3-git-send-email-allen.chen@ite.com.tw