ARM: dts: exynos: correct PMIC interrupt trigger level on Monk

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: e0cefb3f ("ARM: dts: add board dts file for Exynos3250-based Monk board")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201210212903.216728-2-krzk@kernel.org