dt-bindings: display: ti,am65x-dss: Add am625 dss compatible

Add ti,am625-dss compatible string.
The DSS IP on TI's AM625 SoC is an update from the DSS on TI's AM65X
SoC. The former has an additional OLDI TX to enable a 2K resolution on
OLDI displays or enable 2 duplicated displays with a smaller resolution.

Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>
Reviewed-by: Rahul T R <r-ravikumar@ti.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>