dt-bindings: ili9881c: add rotation property

Allow the standard rotation property from panel-common for ILI9881C
based panels.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020153432.383845-3-john@metanate.com