arch: multilevel_irq: fix interrupt bits check

The bits allocated for each aggregator level only need to be enough to
encode CONFIG_MAX_IRQ_PER_AGGREGATOR, instead of the combined number of
IRQs from all aggregators in that level.

Add additional check for L3 interrupts as well, if it is enabled.

Updated the assert in `z_get_sw_isr_table_idx()` to be more verbose.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>