arm64: dts: imx8mq: fix SAI compatible

The i.MX8M SAI block is not compatible with the i.MX6SX one, as the
register layout has changed due to two version registers being added
at the beginning of the address map. Remove the bogus compatible.

Fixes: 8c61538d

 ("arm64: dts: imx8mq: Add SAI2 node")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>