[PATCH v3] mfd: tps6586x: add basic interrupt support

[Gary King]

Hi Samuel, Mike,

This patch incorporates Mike's feedback to my v2 patch to add interrupt
support to the tps6586x driver.

Changes since v2:
 * Remove stale comment in commit message

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.

Thanks,

Gary

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