dt-bindings: gpio: pca95xx: add entry for pca6408

The NXP PCA5608 is the 8-bit version of PCA5616.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>