PCI: dwc: Fix inner MSI IRQ domain registration

PCI: dwc: Fix inner MSI IRQ domain registration

On a system that uses the internal DWC MSI widget, I get this
warning from debugfs when CONFIG_GENERIC_IRQ_DEBUGFS is selected:

  debugfs: File ':soc:pcie@fc000000' in directory 'domains' already present!

This is due to the fact that the DWC MSI code tries to register two
IRQ domains for the same firmware node, without telling the low
level code how to distinguish them (by setting a bus token). This
further confuses debugfs which tries to create corresponding
files for each domain.

Fix it by tagging the inner domain as DOMAIN_BUS_NEXUS, which is
the closest thing we have as to "generic MSI".

Link: https://lore.kernel.org/r/20200501113921.366597-1-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>