docs: arm64: gic-v5: Document booting requirements for GICv5

Document the requirements for booting a kernel on a system implementing
a GICv5 interrupt controller.

Specifically, other than DT/ACPI providing the required firmware
representation, define what traps must be disabled if the kernel is
booted at EL1 on a system where EL2 is implemented.

Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250703-gicv5-host-v7-30-12e71f1b3528@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>