drm/omap: Prepare DSS for probing without legacy platform data

In order to probe display subsystem (DSS) components with ti-sysc
interconnect target module without legacy platform data and using
devicetree, we need to update dss probing a bit.

In the device tree, we will be defining the data also for the interconnect
target modules as DSS really is a private interconnect. There is some
information about that in 4460 TRM in "Figure 10-3. DSS Integration" for
example where it mentions "32-bit interconnect (SLX)".

The changes we need to make are:

1. Parse also device tree subnodes for the compatible property fixup

2. Update the component code to consider device tree subnodes

Cc: dri-devel@lists.freedesktop.org
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>