The hardware don't have a SAI4 instance so remove the define. Use a
comment to keep it as reference and to avoid confusion.

Fixes: 10886914 ("dt-bindings: imx: Add clock binding doc for i.MX8MP")
Acked-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://lore.kernel.org/r/20230731142150.3186650-2-m.felsch@pengutronix.deSigned-off-by: Abel Vesa <abel.vesa@linaro.org>