gpio: ep93xx: Cut gpio_to_irq() usage

This removes the callback into the gpiolib creating a
circular call to convert between GPIO numbers and IRQs
and pushes the whole business into the driver, just
using an array of IRQ bases for the three IRQ capable
ports.

This way we get rid of including <linux/gpio.h> that
no driver should include.

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