irqchip/armada-370-xp: Document the overall driver logic
Since the overall logic of the driver to handle the global and per-CPU
masking of the interrupts is far from trivial, this commit adds a long
comment detailing how the hardware operates and what strategy the
driver implements on top of that.
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>