ARM: dts: imx6qdl-gw551x: Do not use 'simple-audio-card,dai-link'

According to Documentation/devicetree/bindings/sound/simple-card.txt
the 'simple-audio-card,dai-link' may be omitted when the card has
only one DAI link, which is the case here.

Get rid of 'simple-audio-card,dai-link' in order to fix the following
build warning with W=1:

arch/arm/boot/dts/imx6qdl-gw551x.dtsi:109.32-121.5: Warning (unit_address_vs_reg): /sound-digital/simple-audio-card,dai-link@0: node has a unit name, but no reg property

Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>