Support SW stats with debugfs.

Query output:
$ cat /sys/kernel/debug/hns_roce/hns_0/sw_stat/sw_stat
aeqe                 --- 3341
ceqe                 --- 0
cmds                 --- 6764
cmds_err             --- 0
posted_mbx           --- 3344
polled_mbx           --- 3
mbx_event            --- 3341
qp_create_err        --- 0
qp_modify_err        --- 0
cq_create_err        --- 0
cq_modify_err        --- 0
srq_create_err       --- 0
srq_modify_err       --- 0
xrcd_alloc_err       --- 0
mr_reg_err           --- 0
mr_rereg_err         --- 0
ah_create_err        --- 0
mmap_err             --- 0
uctx_alloc_err       --- 0
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Link: https://lore.kernel.org/r/20231114123449.1106162-4-huangjunxian6@hisilicon.comSigned-off-by: Leon Romanovsky <leon@kernel.org>