pinctrl: pinconf: Add generic helper function for freeing mappings

The pinconf-generic.h file exposes functions for creating generic mappings
but it does not expose a function for freeing the mappings. Add a function
for freeing generic mappings.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>