dt-bindings: pinctrl: xilinx: Add support for function with pins

Support specifying the function per-pin. The driver doesn't care
whether you use pins or groups for this purpose.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20240610223550.2449230-2-sean.anderson@linux.dev
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: Sean Anderson