dt-bindings: serial: fsl-lpuart: Add i.MX8DXL compatible

Add i.MX8DXL lpuart compatible to the bindings documentation.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20220118131715.2051965-1-abel.vesa@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>