dt-bindings: display: bridge: Add YAML schema for Synopsys DW-HDMI

Add a .yaml schema containing the common properties for the Synopsys
DesignWare HDMI TX controller. This isn't a full device tree binding
specification, but is meant to be referenced by platform-specific
bindings for the IP core.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>