of/irq: Split of_msi_map_rid to reuse msi-map lookup

The msi-map property is also used to identify the MSI controller
as a form of grown-up msi-parent property.

Looking it up is complicated enough, and since of_msi_map_rid
already implements this, let's turn it into an internal utility
function. We'll put that to good use later on.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>