devicetree: encode multi-level interrupt number in C devicetree magic

The multi-level encoding of the interrupt number currently
happens in the `gen_defines.py`, which is called in the
`dts.cmake` module after `kconfig.cmake`. However, the number
of bits used by each level is defined in Kconfig and this means
that `gen_defines.py` will not be able to get that information
during build.

To fix this, do the multi-level encoding in C devicetree macro
magic instead of the python script. This ticks one of a
long-standing TODO item from the `gen_defines.py`.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>