Amend the Serial device bus Kconfig entries to clarify that you most
likely also want to enable TTY port controller support, and make
SERIAL_DEV_CTRL_TTYPORT default to Y (when bus support is enabled).

Note that the TTY port controller is currently the only in-kernel
serdev controller implementation.
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>