dt-bindings: PCI: pci-ep: Add support for iommu-map and msi-map

Document the use of (msi|iommu)-map for PCI Endpoint (EP) controllers,
which can use MSI as a doorbell mechanism. Each EP controller can support
up to 8 physical functions and 65,536 virtual functions.

Define how to construct device IDs using function bits [2:0] and virtual
function index bits [31:3], enabling (msi|iommu)-map to associate each
child device with a specific (msi|iommu)-specifier.

The EP cannot rely on PCI Requester ID (RID) because the RID is determined
by the PCI topology of the host system. Since the EP may be connected to
different PCI hosts, the RID can vary between systems and is therefore not
a reliable identifier.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/all/20250414-ep-msi-v18-4-f69b49917464@nxp.com