Input: wm831x - add driver for Wolfson WM831x PMIC touchscreen controllers

Some of the WM831x series of PMICs from Wolfson Microelectronics include
a resistive touchscreen controller. Implement support for these controllers
within the input API.

Platform data is supported to allow configuration of system parameters such
as selection between four and five wire touchscreens and for specification
of optional direct to CPU IRQs for sample availability and for pen down.
Use of this feature for at least the data IRQ is strongly recommended.

Thanks to Julien Boibessot for extensive testing and detailed feedback.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Tested-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>