dt-bindings: input: touchscreen: edt-ft5x06: Add FocalTech FT3518

dt-bindings: input: touchscreen: edt-ft5x06: Add FocalTech FT3518

Document FocalTech FT3518 support by adding the compatible.

Co-developed-by: Kamil Gołda <kamil.golda@protonmail.com>
Signed-off-by: Kamil Gołda <kamil.golda@protonmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Yedaya Katsman <yedaya.ka@gmail.com>
Link: https://patch.msgid.link/20260118-touchscreen-patches-v3-1-1c6a729c5eb4@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>