gpio: ep93xx: refactor ep93xx_gpio_add_bank

- replace plain numbers with girq->num_parents in devm_kcalloc
- replace plain numbers with girq->num_parents for port F
- refactor i - 1 to i + 1 to make loop more readable
- combine getting IRQ's loop and setting handler's into single loop

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>