dt-bindings: display/panel: Add the Focaltech gpt3

Add bindings for the Forcaltech gpt3, which is a 640x480 3.0" 4:3
IPS LCD Panel found in the YLM/Anbernic RG300X handheld.

Signed-off-by: Christophe Branchereau <cbranchereau@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219195233.375637-3-cbranchereau@gmail.com