dt-bindings: arm: fsl: add NXP S32G2 boards

Add bindings for S32G2's evaluation board (S32G-VNP-EVB) and reference
design board 2 ( S32G-VNP-RDB2).

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