auxdisplay: linedisp: support attribute attachment to auxdisplay devices

auxdisplay: linedisp: support attribute attachment to auxdisplay devices

Enable linedisp library integration into existing kernel devices (like LED
class) to provide a uniform 7-segment userspace API without creating
separate child devices, meeting the consistent interface while maintaining
coherent device hierarchies.

This allows uniform 7-segment API across all drivers while solving device
proliferation and fragmented userspace interfaces.

The sysfs attributes appear in one of the two locations depending on usage:
  1. On linedisp.N child devices (legacy linedisp_register())
  2. On the parent auxdisplay device (new linedisp_attach())
Functionality is identical in both modes.

Existing consumers of linedisp_register() are unaffected. The new API
enables drivers like TM16XX to integrate 7-segment display functionality
seamlessly within their LED class device hierarchy.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>