proc: Add nbcon support for /proc/consoles

proc: Add nbcon support for /proc/consoles

Update /proc/consoles output to show 'W' if an nbcon console is
registered. Since the write_thread() callback is mandatory, it
enough just to check if it is an nbcon console.

Also update /proc/consoles output to show 'N' if it is an
nbcon console.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-14-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>