docs: driver-model: document driver_override

When a driver is removed, the list of devices that it supports is

iterated over, and the driver's remove callback is called for each

one. The device is removed from that list and the symlinks removed.

Driver Override

~~~~~~~~~~~~~~~

Userspace may override the standard matching by writing a driver name to

a device's ``driver_override`` sysfs attribute.  When set, only a driver

whose name matches the override will be considered during binding.  This

bypasses all bus-specific matching (OF, ACPI, ID tables, etc.).

The override may be cleared by writing an empty string, which returns

the device to standard matching rules.  Writing to ``driver_override``

does not automatically unbind the device from its current driver or

make any attempt to load the specified driver.

Buses opt into this mechanism by setting the ``driver_override`` flag in

their ``struct bus_type``::

  const struct bus_type example_bus_type = {

      ...

      .driver_override = true,

  };

When the flag is set, the driver core automatically creates the

``driver_override`` sysfs attribute for every device on that bus.

The bus's ``match()`` callback should check the override before performing

its own matching, using ``device_match_driver_override()``::

  static int example_match(struct device *dev, const struct device_driver *drv)

  {

      int ret;

      ret = device_match_driver_override(dev, drv);

      if (ret >= 0)

          return ret;

      /* Fall through to bus-specific matching... */

  }

``device_match_driver_override()`` returns > 0 if the override matches

the given driver, 0 if the override is set but does not match, or < 0 if

no override is set at all.

Additional helpers are available:

- ``device_set_driver_override()`` - set or clear the override from kernel code.

- ``device_has_driver_override()`` - check whether an override is set.