Add support for the per-port interrupt controller that deals with both INTx
signalling and management interrupts.

This allows the Link-up/Link-down interrupts to be wired, allowing the
bring-up to be synchronised (and provide debug information).  The framework
can further be used to handle the rest of the per port events if and when
necessary.

Likewise, INTx signalling is implemented so that end-points can actually be
used.

Link: https://lore.kernel.org/r/20210929163847.2807812-7-maz@kernel.org
Link: https://lore.kernel.org/r/20211004150552.3844830-1-maz@kernel.orgTested-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>