Added new test for `DT_IRQ_LEVEL` and `DT_INST_IRQ_LEVEL`.
Introduced a new `vnd.cpu-intc` compatible so that we have a
root level interrupt controller that acts as level 1
aggregator, and modified `test_intc` to be level 2 aggregator,
updated test of `DT_IRQN(TEST_I2C_BUS)` accordingly.
Signed-off-by: Yong Cong Sin <ycsin@meta.com>
