gpio: zynq: Used platdata structure for storing static data instead of priv

This patch used platdata structure instead of priv for storing static
information read from DT.

Signed-off-by: Vipul Kumar <vipul.kumar@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>