mfd: Export ISEL values from WM831x core

The current settings which can be used with the WM831x current sinks
can't easily be mapped between register values and currents at run
time without a lookup table since the values scale logarithmically
to match the way the human eye interprets brightness. This lookup
table is inclided in the core since several drivers need to use it.

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