gpio: pca953xx: Add support for pca6408

Add support for pca6408 which is the 8-bit version of the pca6416.

https://www.nxp.com/docs/en/data-sheet/PCA6408A.pdf



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