ARM: dts: exynos: correct PMIC interrupt trigger level on Artik 5

The Samsung PMIC datasheets describe the interrupt line as active low
with a requirement of acknowledge from the CPU.  Without specifying the
interrupt type in Devicetree, kernel might apply some fixed
configuration, not necessarily working for this hardware.

Fixes: b004a34b ("ARM: dts: exynos: Add exynos3250-artik5 dtsi file for ARTIK5 module")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20201210212903.216728-1-krzk@kernel.org