scripts/dts: Generate IRQ values for multi-level IRQs

When we have multi-level (ie chained interrupt controllers) Zephyr has a
schemee to encode the multi-level and IRQ values along the level's into
a single 32-bit value.  This is the "IRQ" value expected by Zephyr APIs.

The encoding scheme is documented here:

doc/reference/kernel/other/interrupts.rst

Update the device tree generation to walk the interrupt levels and
generate the expected encoded value for the IRQ.

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