Input: add support for touchscreen on W90P910 ARM platform

The touchscreen works in two modes, wait trigger mode and auto-semi
mode. The device starts in wait trigger mode and waits until pressure
is detected, then device sets WT_INT bit and raises an interrupt.
The driver should put the device into auto-semi mode and prepare for
reading first X and then Y coordinates. When coordinate data is ready
the driver sets ADC_INT bit and raises interrupt again.

[dtor@mail.ru: various cleanups]
Signed-off-by: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>