The code deployed to implement GSI linux IRQ numbers mapping on arm64 turns
out to be generic enough so that it can be moved to ACPI core code along
with its respective config option ACPI_GENERIC_GSI selectable on
architectures that can reuse the same code.

Current ACPI IRQ mapping code is not integrated in the kernel IRQ domain
infrastructure, in particular there is no way to look-up the
IRQ domain associated with a particular interrupt controller, so this
first version of GSI generic code carries out the GSI<->IRQ mapping relying
on the IRQ default domain which is supposed to be always set on a
specific architecture in case the domain structure passed to
irq_create/find_mapping() functions is missing.

This patch moves the arm64 acpi functions that implement the gsi mappings:

acpi_gsi_to_irq()
acpi_register_gsi()
acpi_unregister_gsi()

to ACPI core code. Since the generic GSI<->domain mapping is based on IRQ
domains, it can be extended as soon as a way to map an interrupt
controller to an IRQ domain is implemented for ACPI in the IRQ domain
layer.

x86 and ia64 code for GSI mappings cannot rely on the generic GSI
layer at present for legacy reasons, so they do not select the
ACPI_GENERIC_GSI config options and keep relying on their arch
specific GSI mapping layer.

Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>