dt-bindings: Add vendor prefix for Silicon Linux.

Add Silicon Linux to the list of devicetree vendor prefixes.

Website: http://www.si-linux.co.jp



Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>