pinctrl: zynqmp: Unify pin naming

Since we have devm_kasprintf_strarray() helper, which is used in
the rest of pin control drivers, it makes sense to switch this
driver to it. The pin names are not part of any ABI and hence
there will be no regression based on that. Otherwise all generated
pin names will follow the same schema in the pin control subsystem.

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