pinctrl: single: Use generic parser and #pinctrl-cells for pinctrl-single,pins

We can now use generic parser. To support the legacy binding without
#pinctrl-cells, add pcs_quirk_missing_pinctrl_cells() and warn about
missing #pinctrl-cells.

Let's also update the documentation for struct pcs_soc_data while at it
as that seems to be out of date.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>