gpio: ep93xx: Rename has_debounce to has_irq

This is closer to what the variable (per bank) actually
means. We have the .gpio_to_irq() hook registered only
when this is true.

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