sensors: convert fifo_wm to batch_dur

Having a % FIFO watermark isn't very useful as it doesn't convey how long
the SoC can sleep (or do other work) while batching sensor data. Convert
the attribute to a batch duration using ticks. Currently the ticks are
in system ticks, but eventually when an external clock is attached to
the sensor it will be in the external clock's ticks.

Signed-off-by: Yuval Peress <peress@google.com>