dt-bindings: arm: renesas: Add si-linux cat87[45] boards

This patch adds board cat874 (powered by the RZ/G2E) and board
cat875 (that sits on top of cat874). Both boards are made by
Silicon Linux.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>