pinctrl: stricten up generic group code

Rename the symbol PINCTRL_GENERIC to PINCTRL_GENERIC_GROUPS since
it all pertains to groups. Replace everywhere.

ifdef out the radix tree and the struct when not using the
generic groups.

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