dt-bindings: gpio: gpio-xilinx: Add clk support to xilinx soft gpio IP

Specify clock property in binding.

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/1605201148-4508-3-git-send-email-srinivas.neeli@xilinx.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>