irqchip: Make irqchip_init() usable on pure ACPI systems

Pure ACPI systems (e.g., LoongArch) do not need OF_IRQ, but still
require irqchip_init() to perform the ACPI irqchip probing,
even when OF_IRQ isn't selected.

Relax the dependency to enable the generic irqchip support when
ACPI_GENERIC_GSI is configured.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
[maz: revamped commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220927124557.3246737-1-chenhuacai@loongson.cn