iio: accel: adxl345: Expose IIO_EV_INFO_VALUE for double tap

The ADXL345 uses a single hardware register (ADXL345_REG_THRESH_TAP) to
store the threshold for both single tap and double tap events.

Currently, the driver only exposes the IIO_EV_INFO_VALUE attribute for
the single tap event.

However, the IIO ABI dictates that if an event is supported, its
associated configuration attributes should be exposed to userspace. This
applies even if writing to one channel property alters the value of
another due to shared underlying hardware state.

Add IIO_EV_INFO_VALUE to the double tap event specification to ensure
full ABI compliance.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Taha Ed-Dafili <0rayn.dev@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>