of/irq: Add new function of_msi_map_rid()

The device tree property "msi-map" specifies how to create the PCI
requester id used in some MSI controllers.  Add a new function
of_msi_map_rid() that finds the msi-map property and applies its
translation to a given requester id.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>