This change adds audio jack injection feature through debugfs, with
this feature, we could validate alsa userspace changes by injecting
plugin or plugout events to the non-phantom audio jacks.

With this change, the sound core will build the folders
$debugfs_mount_dir/sound/cardN if SND_DEBUG and DEBUG_FS are enabled.
And if users also enable the SND_JACK_INJECTION_DEBUG, the jack
injection nodes will be built in the folder cardN like below:

$tree $debugfs_mount_dir/sound
$debugfs_mount_dir/sound
├── card0
│   ├── HDMI_DP_pcm_10_Jack
│   │   ├── jackin_inject
│   │   ├── kctl_id
│   │   ├── mask_bits
│   │   ├── status
│   │   ├── sw_inject_enable
│   │   └── type
...
│   └── HDMI_DP_pcm_9_Jack
│       ├── jackin_inject
│       ├── kctl_id
│       ├── mask_bits
│       ├── status
│       ├── sw_inject_enable
│       └── type
└── card1
    ├── HDMI_DP_pcm_5_Jack
    │   ├── jackin_inject
    │   ├── kctl_id
    │   ├── mask_bits
    │   ├── status
    │   ├── sw_inject_enable
    │   └── type
    ...
    ├── Headphone_Jack
    │   ├── jackin_inject
    │   ├── kctl_id
    │   ├── mask_bits
    │   ├── status
    │   ├── sw_inject_enable
    │   └── type
    └── Headset_Mic_Jack
        ├── jackin_inject
        ├── kctl_id
        ├── mask_bits
        ├── status
        ├── sw_inject_enable
        └── type

The nodes kctl_id, mask_bits, status and type are read-only, users
could check jack or jack_kctl's information through them.

The nodes sw_inject_enable and jackin_inject are directly used for
injection. The sw_inject_enable is read-write, users could check if
software injection is enabled or not on this jack, and users could
echo 1 or 0 to enable or disable software injection on this jack. Once
the injection is enabled, the jack will not change by hardware events
anymore, once the injection is disabled, the jack will restore the
last reported hardware events to the jack. The jackin_inject is
write-only, if the injection is enabled, users could echo 1 or 0 to
this node to inject plugin or plugout events to this jack.

For the detailed usage information on these nodes, please refer to
Documentation/sound/designs/jack-injection.rst.
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://lore.kernel.org/r/20210127085639.74954-2-hui.wang@canonical.comSigned-off-by: Takashi Iwai <tiwai@suse.de>