Exar XR17V352/354/358 chips have 16 multi-purpose inputs/outputs which
can be controlled using gpio interface.

Add the gpio specific code.
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>