arm64: dts: zii-ultra: only trigger IRQ on falling edge ucs1002 ALERT pin

The ALERT signaling happens on the falling edge of the signal, as rising
edge doesn't really have any notion, as it may happen much later (due to
shared IRQ line) or too early if the chip resolves the fault itself. So
only trigger the IRQ on the edge we are actually interested in.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>