pinctrl: Introduce struct pinfunction and PINCTRL_PINFUNCTION() macro

There are many pin control drivers define their own data type for
pin function representation which is the same or embed the same data
as newly introduced one. Provide the data type and convenient macro
for all pin control drivers.

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