This patch adds following changes to support tap feature:
- defines pulse event related registers
- enables and handles single pulse interrupt for fxls8471
- handles IIO_EV_DIR_EITHER in read/write callbacks
because event direction for pulse is either rising or
falling.
- configures read/write event value for pulse latency register
using IIO_EV_INFO_HYSTERESIS.
- add multiple events like pulse and tranient event spec
as elements of event_spec array named 'mma8452_accel_events'
Except mma8653 chip all other chips like mma845x and
fxls8471 have single tap detection feature.
Tested thoroughly using iio_event_monitor application on
imx6ul-evk board.