gpio: max732x: convert to GPIOLIB_IRQCHIP

Take a sweep to bring the irq support for the MAX732x expanders
into the gpiolib core to cut down on duplicated code.

Only compile tested! I need some feedback from people using this
expander with interrupts to tell me if things go right or
wrong when I do this.

Cc: Semen Protsenko <semen.protsenko@globallogic.com>
Cc: Mans Rullgard <mans@mansr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>