arm64: dts: exynos: correct PMIC interrupt trigger level on Espresso

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: 9589f772 ("arm64: dts: Add S2MPS15 PMIC node on exynos7-espresso")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201210212903.216728-8-krzk@kernel.org