mfd: Convert WM835x IRQ handling to use a data table

Rather than open coding individual IRQs in each function which
manipulates them store data for IRQs in a table which is then
referenced in the users.

This is a substantial code shrink and should be a performance win in
cases where only a single IRQ goes off at once since instead of
reading four of the second level IRQ registers for each interrupt
we read only the sub-registers which have had an interrupt flagged.

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