irqchip/sunxi-nmi: Document interrupt disabling and clearing at probe time

The sunxi-nmi disables all its interrupts and clears any pending
interrupts at probe time.

Add comments documenting it, just to make it clear.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>