dt-bindings: iio: accel: adxl345: document second interrupt

dt-bindings: iio: accel: adxl345: document second interrupt

The pinout of all the supported chips in this binding have two interrupt
pins. Document the second one, too, even though the Linux driver
currently does not support the second interrupt. Boards may have it
wired nonetheless. While here, drop the dependency of interrupt-names
which is already described in the core.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>