Add FIFO event type:
* IIO_EV_TYPE_FIFO_FLUSH
This change also adds FIFO event directions:
* IIO_EV_DIR_FIFO_EMPTY (indicates the FIFO is empty)
* IIO_EV_DIR_FIFO_DATA (indicates the FIFO is not empty)
A FIFO flush event is triggered after the HW FIFO is flushed,
and indicates that whether the HW FIFO is empty or not.
Signed-off-by: Jason Zhang <jason.zhang@rock-chips.com>
Change-Id: Idb21bc271c7fd8def63f2cb29440ebf0677688d5