gen_isr_tables: remove hard-coded max IRQ value of 32

In the MULTI_LEVEL_INTERRUPTS Kconfig we have a symbol for defining
the maximum IRQ per aggregator: MAX_IRQ_PER_AGGREGATOR

Instead of using a hard-coded value of 32 max irq per level,
let's use the value of MAX_IRQ_PER_AGGREGATOR

Signed-off-by: Michael Scott <mike@foundries.io>