api: irq: Add Multi-level IRQ utility functions

This adds utility functions to irq.h in accordance with zephyr's
multi-level irq numbering schema. Functions that are added will
get the zephyr IRQs level and provide function to return the
interrupt number at a particular level.

Fixes issue #20338

Signed-off-by: Jaron Kelleher <jkelleher@fb.com>