irqchip: armada-370-xp: slightly cleanup irq controller driver

In preparation for the introduction of MSI support in the IRQ
controller driver, we clarify the implementation of IPI using
additional defines for the manipulation of doorbells. Just like IPIs
are implemented using doorbells, MSIs will also use doorbells, so it
makes sense to do this preparatory cleanup first.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>