dt-bindings: display: panel: Document the rotation property

The Jadard jd9365da-h3 driver already allows DRM to get the panel
orientation via the device tree rotation property (described in
panel-common.yaml), but it's currently not documented.
Describe it in the driver documentation to fix a dtbs_check error in
Xiaomi Mi Smart Clock x04g, where the panel is landscape-oriented.

Signed-off-by: Luca Leonardo Scorcia <l.scorcia@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patch.msgid.link/20260309180353.8220-1-l.scorcia@gmail.com