tests: devicetree: check L2 interrupt encoding

Add some assert statements meant to check if L2 interrupts
are encoded right when dealing with nodes that consume interrupts
from multiple aggregators. For this to work, also add another
interrupt controller node which extends a different L1 interrupt
from `test_intc`.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>