iPod Classic: s5l8702 GPIO interrupt controller.

This patch implements a simple API to use the external interrupt
hardware present on s5l8702 (GPIO interrupt controller). This
GPIOIC has been fully tested using emcore apps.

Code is based on openiBoot project, there are a few modifications
to optimize space considering we will only use two or three external
interrupts. The API compiles and works, but has been never used,
therefore probably will need some changes to the final version.

External interrupts are necessary for jack remote+mic controller
(see iAP Interface Specifiction: Headphone Remote and Mic System),
this controller is located at I2C bus address 0x72, there is a IRQ
line for remote button press/release events routed to GPIO E6. At
this moment, the functionallity of this controller has been
extensively tested using emcore, getting a lot of information about
how it works. Microphone is already working on RB, jack accessory
detection and button events are work in progress.

PMU IRQ line is also routed to GPIO F3, it signals many events:
holdswitch, usb plug, wall adapter, low battery... The use of PMU
interrupts is the orthodox way of doing things, at this moment
there is no work done in this direction, there are a lot of PMU
events and i think it is a matter of discursion what to do and how.

Change-Id: Icc2e48965e664ca56c9518d84a81c9d9fdd31736

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