gpio: etraxfs: add support for ARTPEC-3

Add support for the GIO block in the ARTPEC-3 SoC.  The basic
functionality is essentialy the same as the version in the ETRAX FS,
except for a different set of ports, including a read-only port.

Cc: devicetree@vger.kernel.org
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>