irqchip/gic-v2m: Add support for Amazon Graviton variant of GICv3+GICv2m

Add support for Amazon Graviton custom variant of GICv2m, where the message
is encoded using the MSI message address, as opposed to standard
GICv2m, where the SPI number is encoded in the MSI message data.

In addition, the Graviton flavor of GICv2m is used along GICv3 (and not
GICv2).

Co-developed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Zeev Zilberman <zeev@amazon.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>