Add BIST (built in self test) support for phy loopback.

Through the new debugfs interface, the user can configure loopback
mode/linkrate/phy id/code mode before enabling it. And also user can
enable/disable BIST function.

Link: https://lore.kernel.org/r/1567774537-20003-13-git-send-email-john.garry@huawei.comSigned-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>