arm64: dts: zx296718-evb: use audio-graph-card for HDMI audio

Instead of simple-audio-card, audio-graph-card is recommended for audio
bindings.  Let's change to it, so that the HDMI/SPDIF audio card can
align with the new I2S/Codec card which will be added later.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>