gpio: zynq: Add support for suspend resume

Add support for suspend resume. Now that we can lose context across
a suspend/ resume cycle. Add support for the context restore.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>