dt-bindings: timer: nuvoton: Clarify that interrupt of timer 0 should be specified

The NPCM750 Timer/Watchdog Controller has multiple interrupt lines,
connected to multiple timers. The driver uses timer 0 for timer
interrupts, so the interrupt line corresponding to timer 0 should be
specified in DT.

I removed the mention of "flags for falling edge", because the timer
controller uses high-level interrupts rather than falling-edge
interrupts, and whether flags should be specified is up the interrupt
controller's DT binding.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210108163004.492649-1-j.neuschaefer@gmx.net