ARM: dts: armada-38x: Fix irq type for pca955

ARM: dts: armada-38x: Fix irq type for pca955

As written in the datasheet the PCA955 can only handle low level irq and
not edge irq.

Without this fix the interrupt is not usable for pca955: the gpio-pca953x
driver already set the irq type as low level which is incompatible with
edge type, then the kernel prevents using the interrupt:

"irq: type mismatch, failed to map hwirq-18 for
/soc/internal-regs/gpio@18100!"

Fixes: 928413bd859c ("ARM: mvebu: Add Armada 388 General Purpose
Development Board support")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>