mfd: use a dedicated workqueue for pcf50633 irq processing

Using the default kernel "events" workqueue causes problems with
synchronous adc readings if initiated from some task on the same
workqueue.

I had a deadlock trying to use pcf50633_adc_sync_read from a
power_supply class driver because the reading was initiated from the
workqueue and it waited for the irq processing to complete (to get the
result) and that was put on the same workqueue.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>