gpio: ep93xx: Use the hwirq and port

In the IRQ-related functions, switch to using the hwirq
and port number found from the current struct gpio_chip *

As the lower 3 bits of the IRQ number is identical to the
lower 3 bits of the GPIO number we can cut some corners.

Call directly into the gpiochip to set up the direction
and read the input instead of using the consumer API.

This enabled us to cut the confusing irq_to_gpio() macro
that is a remnant of the old generic GPIO API as well.

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