pinctrl: core: Make dt_free_map optional

If the pin controller driver is using devm_kzalloc, there may not be
anything to do for dt_free_map. Let's make it optional to avoid
unncessary boilerplate code.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>