samples: rust: Add debugfs sample driver

Adds a new sample driver that demonstrates the debugfs APIs.

The driver creates a directory in debugfs and populates it with a few
files:
- A read-only file that displays a fwnode property.
- A read-write file that exposes an atomic counter.
- A read-write file that exposes a custom struct.

This sample serves as a basic example of how to use the `debugfs::Dir`
and `debugfs::File` APIs to create and manage debugfs entries.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250904-debugfs-rust-v11-5-7d12a165685a@google.com


[ Change ACPI ID "LNUXDEBF" to "LNUXBEEF". - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>