pinctrl: keembay: comment process of building functions a bit

This should make code a bit easier to follow. While at it use some "for"
loops to simplify array iteration loops.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20211216162206.8027-2-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>