iio: Add event enums for running period and count

There are devices (such as Murata IRS-D200 PIR proximity sensor) that
check the data signal with a running period. I.e. for a specified time,
they count the number of conditions that have occurred, and then signal
if that is more than a specified amount.

`IIO_EV_INFO_PERIOD` resets when the condition no longer is true and is
therefore not suitable for these devices. Add a new `iio_event_info`
`IIO_EV_INFO_RUNNING_PERIOD` that can be used as a running period. Also
add a new `IIO_EV_INFO_RUNNING_COUNT` that can be used to specify the
number of conditions that must occur during this running period.

Signed-off-by: Waqar Hameed <waqar.hameed@axis.com>
Link: https://lore.kernel.org/r/ee4a801ae9b9c4716c7bd23d8f79f232351df8bd.1689753076.git.waqar.hameed@axis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>