hw/intc/loongarch_pch: Rename memory region iomem32_low with iomem

hw/intc/loongarch_pch: Rename memory region iomem32_low with iomem

Rename memory region iomem32_low with iomem, also change ops name
as follows:
  loongarch_pch_pic_reg32_low_ops  --> loongarch_pch_pic_ops
  loongarch_pch_pic_low_readw      --> loongarch_pch_pic_read
  loongarch_pch_pic_low_writew     --> loongarch_pch_pic_write

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20250507023754.1877445-3-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>