dt-bindings: auxdisplay: ht16k33: Document LED subnode

Extend the Holtek HT16K33 LED controller Device Tree bindings with an
LED subnode, conforming to the standard LED bindings.

This allows the user to exert more control, like specifying LED color,
function, and/or trigger, to extend LED functionality beyond a simple
display backlight.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>