irqchip: Select downstream irqchip drivers for LoongArch CPU

LoongArch irqchips have a fixed hierarchy which currently can't be
described by ACPI tables, so upstream irqchip drivers call downstream
irqchip drivers' initialization directly. As a result, the top level
(CPU-level) irqchip driver should explicitly select downstream drivers
to avoid build errors.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220808085319.3350111-1-chenhuacai@loongson.cn