Input: ad7879 - add header file to fix ad7879.c build errors

Add header file to fix these build errors:

../drivers/input/touchscreen/ad7879.c: In function 'ad7879_parse_dt':
../drivers/input/touchscreen/ad7879.c:505:2: error: implicit declaration of function 'device_property_read_u32' [-Werror=implicit-function-declaration]
  err = device_property_read_u32(dev, "adi,resistance-plate-x", &tmp);
  ^
../drivers/input/touchscreen/ad7879.c:512:2: error: implicit declaration of function 'device_property_read_u8' [-Werror=implicit-function-declaration]
  device_property_read_u8(dev, "adi,first-conversion-delay",
  ^
../drivers/input/touchscreen/ad7879.c:521:2: error: implicit declaration of function 'device_property_read_bool' [-Werror=implicit-function-declaration]
  ts->swap_xy = device_property_read_bool(dev, "touchscreen-swapped-x-y");
  ^

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>