gpio: xilinx: convert from OF GPIO to standard devm APIs

This driver was using the OF GPIO helper API, but barely used any of its
features and it cost more code than it saved. Also, the OF GPIO code is
now deprecated. Convert it to use a more standard setup and use devm
APIs for initialization to avoid the need for a remove function.

Our rationale for this change is that we are using the Xilinx GPIO with
resources injected using the MFD core rather than on the device tree
itself. Using platform rather than OF-specific resources allows this to
work for free.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>