platform-msi: ACPI: Add MSI fwnode registration function

Similar to PCI, there can be platform devices which need
MSI services. They need to know what is the MSI domain on
ACPI based systems. So, similar to pci-msi, provide a way
in platform-msi to register the MSI controller's fwnode.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>