arm64: dts: imx93: add "fsl,imx8ulp-lpuart" compatible for imx93

i.MX93 and i.MX8ULP lpuart add some new featurs based on i.MX7ULP, for
example, i.MX93 and i.MX8ULP can support EOP(end-of-packet) function
while i.MX7ULP doesn't support, so add "fsl,imx8ulp-lpuart" compatible
string for i.MX93 to support those new features.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>