arm64: dts: imx8m-beacon-kit: Remove extra sound-sai entry

'sound-dai' should contain only 'sai5'. Remove the extra entry to
fix the following schema warning:

imx8mm-beacon-kit.dtb: sound-hdmi: simple-audio-card,cpu:sound-dai: [[110], [0]] is too long
	from schema $id: http://devicetree.org/schemas/sound/simple-card.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>