dt-bindings: display: ti: Add schema for AM625 OLDI Transmitter

The OLDI transmitters (TXes) do not have registers of their own, and are
dependent on the source video-ports (VPs) from the DSS to provide
configuration data. This hardware doesn't directly sit on the internal
bus of the SoC, but does so via the DSS. Hence, the OLDI TXes are
supposed to be child nodes under the DSS, and not independent devices.

Two of the OLDI TXes can function in tandem to output dual-link OLDI
output, or cloned single-link outputs. In these cases, one OLDI will be
the primary OLDI, and the other one, a companion. The following diagram
represents such a configuration.

+-----+-----+ +-------+
| | | | |
| | VP1 +----+--->+ OLDI0 | (Primary - may need companion)
| | | | | |
| DSS +-----+ | +-------+
| | | |
| | VP2 | | +-------+
| | | | | |
+-----+-----+ +--->+ OLDI1 | (Companion OLDI)
| |
+-------+

The DSS in AM625 SoC has a configuration like the one above. The AM625
DSS VP1 (port@0) can connect and control 2 OLDI TXes, to use them in
dual-link or cloned single-link OLDI modes. It is only the VP1 that can
connect to either OLDI TXes for the AM625 DSS, and not the VP2.

Alternatively, on some future TI SoCs, along with the above
configuration, the OLDI TX can _also_ connect to separate video sources,
making them work entirely independent of each other. In this case,
neither of the OLDIs are "companion" or "secondary" OLDIs, and nor do
they require one. They both are independent and primary OLDIs. The
following diagram represents such a configuration.

+-----+-----+ +-------+
| | | | |
| | VP1 +--+----------->+ OLDI0 | (Primary - may need companion)
| | | | | |
| +-----+ | +-------+
| | | |
| | VP2 | |
| | | |
| DSS +-----+ | +---+ +-------+
| | | +-->+ M | | |
| | VP3 +----->+ U +--->+ OLDI1 | (Companion or Primary)
| | | | X | | |
| +-----+ +---+ +-------+
| | |
| | VP4 |
| | |
+-----+-----+

Note that depending on the mux configuration, the OLDIs can either be
working together in tandem - sourced by VP1, OR, they could be working
independently sourced by VP1 and VP3 respectively.
The idea is to support all the configurations with this OLDI TX schema.

The OLDI functionality is further supported by a system-control module,
which contains a few registers to control OLDI IO power and other
electrical characteristics of the IO lanes.

Add devicetree binding schema for the OLDI TXes to support various
configurations, and extend their support to the AM625 DSS.

Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>
Signed-off-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>
Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://lore.kernel.org/r/20250528122544.817829-3-aradhya.bhatia@linux.dev
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>