ARM: dts: mstar: Set gpio compatible for ssd20xd

Now there is gpio support for ssd20xd set the right compatible in the gpio node.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Reviewed-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>