serial: of_serial: Handle auto-flow-control property

Automatic Flow Control capability is not tied to this
property. This is only one way of detecting it. The property
is limited to be used only with 8250 driver.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>