pinctrl: bcm: ns: use generic groups & functions helpers

This simplifies ns driver and gets rid of ~70 lines of code.

"const" had to be dropped from "struct ns_pinctrl_group" @pins to match
"struct group_desc" @pins and pinctrl_generic_add_group(). Otherwise it
would cause:

drivers/pinctrl/bcm/pinctrl-ns.c: In function 'ns_pinctrl_probe':
drivers/pinctrl/bcm/pinctrl-ns.c:277:13: warning: passing argument 3 of 'pinctrl_generic_add_group' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  277 |        group->pins, group->num_pins, NULL);
      |        ~~~~~^~~~~~

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20211222064344.14624-1-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>