irqchip/armada-370-xp: Enable MSI affinity configuration

With multiple devices attached via PCIe to an Armada 385 it is possible
to overwhelm a single CPU with MSI interrupts. Under certain scenarios
configuring the interrupts to be handled by more than one CPU would
prevent the system from being overwhelmed. However the
irqchip-aramada-370-xp driver is configured to only handle MSIs on the
boot CPU, and provides no affinity configuration.

This change adds support to the armada-370-xp driver to allow for
configuring the affinity of specific MSI irqs and to generate the
interrupts on secondary CPUs. This is done by enabling the private
doorbell for all online CPUs and configures all CPUs to unmask MSI
specific private doorbell bits. The CPU affinity selection of the
interrupt is handled by the target list of the software triggered
interrupt value, which is provided as the MSI message. The message has
the associated CPU bit set for the target CPU. For private doorbell
interrupts only one bit can be set otherwise all CPUs will receive the
interrupt, so the lowest CPU in the affinity mask is used. This means
that by default the first CPU will handle all the interrupts as was the
case before.

Signed-off-by: Nathan Rossi <nathan.rossi@digi.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220422043532.146946-1-nathan@nathanrossi.com