scripts: dts: extract: Fix handling of irq prop being a list of lists

Before dtc 1.4.7 we'd get something like the following for an interrupt
property in which #interrupt-cells is 2.

interrupts = <1 2 3 4>;

After dtc 1.4.7 we get:

interrupts = <1 2>, <3 4>;

We should handle both cases in the extract interrupt handling code.  So
if we see a list of lists, we flatten it to a single list to normalize
the property.

Fixes: #9558

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>