mfd: arizona: Interrupt support

Several forthcoming Wolfson devices are based on a common platform
known as Arizona allowing a great deal of reuse of driver code. This
patch adds support for the interrupt controller on Arizona class devices.
Since there are two interrupt domains in the device which share a single
/IRQ pin by default we use two regmap IRQ domains with a trivial demux
interrupt domain used to distribute the interrupts to the two devices.
The devices do support multiple interrupt signals, future work will enable
support for using this feature to avoid the demux.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>