mfd: Add fast path for WM831x touchscreen interrupts

The WM831x interrupt controller provides reporting of the touchscreen
related interrupts in the primary interrupt status register as a
performance optimisation - use this to avoid reading the secondary
status registers for those interrupts.

For code simplicity and to avoid iterating over all interrupts we open
code for the two affected interrupt sources.

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