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

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: 53dd4138 ("ARM: dts: Add exynos5250-spring device tree")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201210212903.216728-4-krzk@kernel.org