devicetree.h: switch to DT_IRQ_INTC_BY_IDX for L2 and L3 INTID encodings

Using `DT_IRQ_INTC()` to fetch the interrupt controller associated
with a node works well for nodes which consume interrupts from a
single aggregator. However, when specifying multiple (and different)
interrupt aggregators via the `interrupts-extended` property,
the L2 and L3 interrupts will no longer be encoded properly. This
is because `DT_IRQ_INTC(node_id)` uses `DT_IRQ_INTC_BY_IDX(node_id, 0)`
so all the interrupts will use the first aggregator as their parent.
To fix this, switch from using `DT_IRQ_INTC()` to `DT_IRQ_INTC_BY_IDX()`.

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