dt-bindings: gpio: msc313: Add compatible for ssd20xd

Add a compatible string for "ssd20xd" for the SigmaStar SSD201
and SSD202D chips. These chips are the same die with different
memory bonded so they don't need their own strings.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>