arm64: dts: imx8qxp-ss-adma: Drop fsl,imx7ulp-lpuart comaptible

The driver differs from clocks point of view, so the i.MX8QXP
is not backwards compatible with i.MX7ULP.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>