edtlib: fix _raw_unit_addr() function

The '_raw_unit_addr()' function was incorrectly written a long time ago.
It tries to look up and use the '#address-cells' of the node containing
the 'interrupts' property itself, instead of using the '#address-cells' of
the interrupt controller's node (or its parent).

As a result, the mapping is successful only if the '#address-cells' value
is the same in both the node and the interrupt controller, all other cases
lead to random errors being generated by the 'gen_defines.py' script.

Also, the function was trying to use the '#address-cells' value as a
little-endian one, which led to unpredictable results as well.
This problem has been fixed too.

Fixes #77890, and probably #78020

Signed-off-by: Andrey VOLKOV <andrey.volkov@munic.io>
Signed-off-by: Ioannis Damigos <ioannis.damigos.uj@renesas.com>