ARM64/irqchip: Update ACPI_IORT symbol selection logic

ACPI IORT is an ACPI addendum to describe the connection topology of
devices with IOMMUs and interrupt controllers on ARM64 ACPI systems.

Currently the ACPI IORT Kbuild symbol is selected whenever the Kbuild
symbol ARM_GIC_V3_ITS is enabled, which in turn is selected by ARM64
Kbuild defaults. This makes the logic behind ACPI_IORT selection a bit
twisted and not easy to follow. On ARM64 systems enabling ACPI the
kbuild symbol ACPI_IORT should always be selected in that it is a kernel
layer provided to the ARM64 arch code to parse and enable ACPI firmware
bindings.

Make the ACPI_IORT selection explicit in ARM64 Kbuild and remove the
selection from ARM_GIC_V3_ITS entry, making the ACPI_IORT selection
logic clearer to follow.

Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>