irq: relocate multi-level irq out of irq.h

Relocate multi-level interrupts APIs out of `irq.h` into
a new file named `irq_multilevel.h` to provide cleaner
separation between typical irq & multilevel ones.

Added preprocessor versions of `irq_to_level_x` as `IRQ_TO_Lx`.

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