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