PCI: mediatek: Advertise lack of built-in MSI handling

Some Mediatek host bridges cannot handle MSIs, which is sad.
This also results in an ugly warning at device probe time,
as the core PCI code wasn't told that MSIs were not available.

Advertise this fact to the rest of the core PCI code by
using the 'msi_domain' attribute, which still opens the possibility
for another block to provide the MSI functionnality.

[maz: commit message, switched over to msi_domain attribute]

Link: https://lore.kernel.org/r/20210330151145.997953-13-maz@kernel.org


Reported-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>