serial: add PORT_GENERIC definition

Current pattern in the linux kernel is that every new serial driver adds
one or more new PORT_ definitions because uart_ops::config_port()
callback documentation prescribes setting port->type according to the
type of port found, or to PORT_UNKNOWN if no port was detected.

When the specific type of the port is not important to the userspace
there's no need for a unique PORT_ value, but so far there's no suitable
identifier for that case.

Provide generic port type identifier other than PORT_UNKNOWN for ports
which type is not important to userspace.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20231008001804.889727-1-jcmvbkbc@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>