irqchip/gicv3-its: platform-msi: Scan MADT to create platform msi domain

With the introduction of its_pmsi_init_one(), support for ACPI
firmware interface can be plugged into the gicv3 ITS driver.

Add code to scan the MADT table to get the ITS entry(ies), then use
the information to create the platform msi domain for devices
connected to it, mirroring the ITS PCI MSI code path.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
[lorenzo.pieralisi@arm.com: rewrote commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Tested-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Wei Xu <xuwei5@hisilicon.com>
Tested-by: Sinan Kaya <okaya@codeaurora.org>
Cc: Tomasz Nowicki <tn@semihalf.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>