dt-bindings: serial: fsl-lpuart: Add imx93 compatible string

The lpuart on i.MX93 is derived from i.MX7ULP with some industrial
enhancements, it uses two compatible strings, so update the
compatible string for i.MX93.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20220323093659.1722463-1-peng.fan@oss.nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>