gpio: aggregator: Replace custom get_arg() with a generic next_arg()

cmdline library provides next_arg() helper to traverse over parameters
and their values given in command line. Replace custom approach in the driver
by it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>