gpio: aggregator: introduce basic configfs interface

gpio: aggregator: introduce basic configfs interface

The existing sysfs 'new_device' interface has several limitations:
* No way to determine when GPIO aggregator creation is complete.
* No way to retrieve errors when creating a GPIO aggregator.
* No way to trace a GPIO line of an aggregator back to its
  corresponding physical device.
* The 'new_device' echo does not indicate which virtual gpiochip<N>
  was created.
* No way to assign names to GPIO lines exported through an aggregator.

Introduce the new configfs interface for gpio-aggregator to address
these limitations. It provides a more streamlined, modern, and
extensible configuration method. For backward compatibility, the
'new_device' interface and its behavior is retained for now.

This commit implements basic functionalities:

  /config/gpio-aggregator/<name-of-your-choice>/
  /config/gpio-aggregator/<name-of-your-choice>/live
  /config/gpio-aggregator/<name-of-your-choice>/dev_name
  /config/gpio-aggregator/<name-of-your-choice>/<lineY>/
  /config/gpio-aggregator/<name-of-your-choice>/<lineY>/key
  /config/gpio-aggregator/<name-of-your-choice>/<lineY>/offset
  /config/gpio-aggregator/<name-of-your-choice>/<lineY>/name

Basic setup flow is:
1. Create a directory for a GPIO aggregator.
2. Create subdirectories for each line you want to instantiate.
3. In each line directory, configure the key and offset.
   The key/offset semantics are as follows:
   * If offset is >= 0:
     - key specifies the name of the chip this GPIO belongs to
     - offset specifies the line offset within that chip.
   * If offset is <0:
     - key needs to specify the GPIO line name.
4. Return to the aggregator's root directory and write '1' to the live
   attribute.

For example, the command in the existing kernel doc:

  echo 'e6052000.gpio 19 e6050000.gpio 20-21' > new_device

is equivalent to:

  mkdir /sys/kernel/config/gpio-aggregator/<custom-name>
  # Change <custom-name> to name of your choice (e.g. "aggr0")
  cd /sys/kernel/config/gpio-aggregator/<custom-name>
  mkdir line0 line1 line2  # Only "line<Y>" naming allowed.
  echo e6052000.gpio > line0/key
  echo 19            > line0/offset
  echo e6050000.gpio > line1/key
  echo 20            > line1/offset
  echo e6050000.gpio > line2/key
  echo 21            > line2/offset
  echo 1             > live

The corresponding gpio_device id can be identified as follows:

  cd /sys/kernel/config/gpio-aggregator/<custom-name>
  ls -d /sys/devices/platform/`cat dev_name`/gpiochip*

Also, via configfs, custom GPIO line name can be set like this:

  cd /sys/kernel/config/gpio-aggregator/<custom-name>
  echo "abc"         > line1/name

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250407043019.4105613-5-koichiro.den@canonical.com
[Bartosz: remove stray newlines]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>