arm64: dts: allwinner: a64: Fix the TCON output clock

Even though we shouldn't really have any external user of the clock
provided by the TCON, if clock-output-names is set, then #clock-cells must
be there as well.

Fix this.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>