pinctrl: pinconf-generic: Convert ..._parse_dt_pinmux() to fwnode API

Convert pinconf_generic_parse_dt_pinmux() to fwnode API. This makes code
cleaner and potentially reusable for some other types of fwnodes, such as
software nodes.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>