irqchip/gic-v2m: Refactor to prepare for ACPI support

This patch replaces the struct device_node with struct fwnode_handle
since this structure is common between DT and ACPI.

It also refactors gicv2m_init_one() to prepare for ACPI support.
The only functional change is removing the node name from pr_info.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>