irqchip/armada-370-xp: Re-order register definitions

In order to clarify to which register base the various register
definitions apply, this commit re-orders them, and adds a comment that
clearly indicate which registers are relative to "main_int_base" and
which registers are relative to "per_cpu_int_base".

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>