dt-bindings: serial: fsl-linflexuart: add compatible for S32G2

Add a compatible string for the uart binding of NXP S32G2 platforms. Here
we use "s32v234-linflexuart" as fallback since the current linflexuart
driver can still work on S32G2.

Signed-off-by: Chester Lin <clin@suse.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>