irq: multilevel: avoid manipulations with bits by using union type

Use a union struct to partition the bits in a
multilevel-encoded interrupt number instead of manipulating
the bits manually, which is tedious and error prone.

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