[PATCH v2] mfd: tps6586x: add basic interrupt support

[Gary King]

Hi Samuel, Mike,

This patch incorporates Mike Rapoport's feedback on my initial patch
to add interrupt support to the tps6586x driver.

Changes since v1:

* Move the interrupt enum ordering to match the ACK register values

* Treat the INT_ACK registers as a single 32b unsigned int.

* Name and define all the interrupts on the device

* If reading the ACK registers in the interrupt thread fails, IRQ_NONE
  is returned, rather than disabling the IRQ handler. This fixes
  interrupts following suspend.

* The cascaded IRQ is passed through the i2c_client's .irq field, rather
  than through platform data.

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