PCI/MSI: Kill msi_controller structure

msi_controller had a good, long life as the abstraction for
a driver providing MSIs to PCI devices. But it has been replaced
in all drivers by the more expressive generic MSI framework.

Farewell, struct msi_controller.

Link: https://lore.kernel.org/r/20210330151145.997953-9-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>