dt-bindings: Add legacy 'toppoly' vendor prefix

The 'toppoly' vendor prefix is in use and refers to TPO, whose DT vendor
prefix is already defined as 'tpo'. Add 'toppoly' as an alternative and
document it as legacy.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813201101.30980-3-laurent.pinchart@ideasonboard.com