gpio: ep93xx: Do not pingpong irq numbers

For setting debounce config we want to write an offset in
a per-gpiochip register, and we know which gpiochip we are
on. Instead of a roundtrip over the IRQ number, figure out
what port we are on for this GPIO chip, then index to the
right register and write the value.

This adds the ep93xx_gpio_port() that finds the port index
from a struct gpio_chip * that we can later exploit to
simplify more code.

Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>