mfd: Support multiple active WM831x AUXADC conversions

The WM831x AUXADC hardware can schedule multiple conversions at once,
allowing higher performance when more than one source is in use as we
can have the hardware start new conversions without having to wait for
a register write.

Take advantage of this in the interrupt driven case, maintaining a list of
callers that are waiting for AUXADC conversions and completing them all
simultaneously. The external interface of the AUXADC is not changed so
there will be limited use of the feature immediately.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>