gpio: aggregator: expose aggregator created via legacy sysfs to configfs

gpio: aggregator: expose aggregator created via legacy sysfs to configfs

Expose settings for aggregators created using the sysfs 'new_device'
interface to configfs. Once written to 'new_device', an "_sysfs.<N>" path
appears in the configfs regardless of whether the probe succeeds.
Consequently, users can no longer use that prefix for custom GPIO
aggregator names. The 'live' attribute changes to 1 when the probe
succeeds and the GPIO forwarder is instantiated.

Note that the aggregator device created via sysfs is asynchronous, i.e.
writing into 'new_device' returns without waiting for probe completion,
and the probe may succeed, fail, or eventually succeed via deferred
probe. Thus, the 'live' attribute may change from 0 to 1 asynchronously
without notice. So, editing key/offset/name while it's waiting for
deferred probe is prohibited.

The configfs auto-generation relies on create_default_group(), which
inherently prohibits rmdir(2). To align with the limitation, this commit
also prohibits mkdir(2) for them. When users want to change the number
of lines for an aggregator initialized via 'new_device', they need to
tear down the device using 'delete_device' and reconfigure it from
scratch. This does not break previous behavior; users of legacy sysfs
interface simply gain additional almost read-only configfs exposure.

Still, users can write to the 'live' attribute to toggle the device
unless it's waiting for deferred probe. So once probe succeeds, they can
deactivate it in the same manner as the devices initialized via
configfs.

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250407043019.4105613-7-koichiro.den@canonical.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>