irqchip/armada-370-xp: Use PCI_MSI_DOORBELL_START where appropriate

As suggested by Gregory Clement, this commit adjusts the
irq-armada-370-xp driver to use the PCI_MSI_DOORBELL_START define in
the armada_370_xp_handle_msi_irq() function, rather than hardcoding
its value.

Suggested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1455115621-22846-4-git-send-email-thomas.petazzoni@free-electrons.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>