pinctrl: keembay: rework loops looking for groups names

Make the outer loop iterate over functions as that's the real subject.
This simplifies code (and reduces amount of lines of code) as allocating
memory for names doesn't require extra checks anymore.

While at it use local "group_names" variable. The plan for
"struct function_desc" is to make its "group_names" /double/ const. That
will allow drivers to use it with static const data.

This keembay "group_names" change is required to avoid:
drivers/pinctrl/pinctrl-keembay.c: In function 'keembay_add_functions':
drivers/pinctrl/pinctrl-keembay.c:1594:8: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1594 |    grp = func->group_names;
      |        ^

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20211216162206.8027-3-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>