dt-bindings: ili9881c: add compatible string for Wanchanglong w552946aba

It utilizes an Ilitek ILI9881D controller chip, but its
compatible with ili9881c so should be added to ilitek,ili9881c file.

Add the compatible string for it.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211016102232.202119-4-michael@amarulasolutions.com