gpio: adp5588.c: Switch to events system

Interupts were generated using GPIN interrupts of
ADP5588. These interrupts have two important limitations:
1. Interrupts can only be generated for either rising or
   falling edges but not both.
2. Interrupts are reasserted as long as the interrupt condition
   persists (i.e. high or low level on that GPIN). This generates
   lots of interrupts unless the event is very short.

To overcome this, ADP5588 provides an event system which queues
up to 10 events in a buffer. GPIN events are queued whenever the
GPIN is asserted or deasserted. This makes it possible to support
generating GPIN interrupts for both edges and to generate only one
interrupt per state change.
Thus it is possible to chain the gpio-keys driver for some GPIOs.

Signed-off-by: Nikolaus Voss <nikolaus.voss@loewensteinmedical.de>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>