Use the new GPIO descriptor interface to handle the zForce GPIOs.
This simplifies the code and allows transparently handle GPIO polarity, as
specified in device tree data.

Also switch to using gpio_{set|get}_value_cansleep() since none of the
callers is in atomic context and cansleep variant allows more GPIO
controllers to be used with the touchscreen.
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>