dt-bindings: auxdisplay: ht16k33: Keyscan function should be optional

Keyscan should be optional to support simple LED matrix displays (output
only).

Reported-by: Michael Kaplan <M.KAPLAN@evva.com>
Signed-off-by: Robin van der Gracht <robin@protonic.nl>
[geert: Rebased]
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>