gpio: ds4520: Add ADI DS4520 GPIO Expander Support

The DS4520 is a 9-bit nonvolatile (NV) I/O expander.
It offers users a digitally programmable alternative
to hardware jumpers and mechanical switches that are
being used to control digital logic node.

Signed-off-by: Okan Sahin <okan.sahin@analog.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>