irq: multilevel: add API to increment a multilevel IRQ

Unlike a normal IRQ, a multilevel IRQ can't be incremented by
simply `irq++`, as that would always increment the L1 of a IRQ,
regardless of its level. A function that understands the level
for which the IRQ operates in is required.

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