dt-bindings: display: bridge: ldb: Add an i.MX6SX entry

i.MX6SX has a single LVDS port and share a similar LDB_CTRL register
layout with i.MX8MP and i.MX93.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404020129.509356-1-festevam@gmail.com