pinctrl: st: Switch to use devm_kasprintf_strarray()

Since we have a generic helper, switch the module to use it.

As a side effect, add check for the memory allocation failures and
cleanup it either in error case or when driver is unloading.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>