of/irq: Make of_msi_map_rid() PCI bus agnostic

There is nothing PCI bus specific in the of_msi_map_rid()
implementation other than the requester ID tag for the input
ID space. Rename requester ID to a more generic ID so that
the translation code can be used by all busses that require
input/output ID translations.

No functional change intended.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200619082013.13661-11-lorenzo.pieralisi@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>