gpio/xilinx: Remove offset property

Instead of calculating the register offset per call, pre-calculate it on
probe time.

Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>