arm64: dts: renesas: r8a774c0: cat874: Add definition for 12V regulator

Power rail "D12.0V" comes straight from the power barrel connector,
and it's used in both main board and sub board.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>