pinctrl: bcm2835: Refactor platform data

This prepares the platform data to be easier to extend for more GPIOs.
Except of this there is no functional change.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1581166975-22949-3-git-send-email-stefan.wahren@i2se.com
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>