irqchip/loongson-pch-lpc: Add dependence on LoongArch

The loongson-pch-lpc driver may be selected in a random
configuration, but it is only supported for LoongArch, So,
the dependence on LoongArch is added for it to avoid compile
error for a random configuration of other architetures.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220916071926.28368-1-lvjianmin@loongson.cn