pinctrl: Introduce generic #pinctrl-cells and pinctrl_parse_index_with_args

Introduce #pinctrl-cells helper binding and generic helper functions
pinctrl_count_index_with_args() and pinctrl_parse_index_with_args().

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
[Forward-declare of_phandle_args]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>