This change adds a flag to indicate that a UART is has an external means
of synchronising its FIFO, without needing CTSRTS or XON/XOFF.

This allows us to use the throttle/unthrottle callbacks, without having
to claim other methods of flow control.
Signed-off-by: NJeremy Kerr <jk@ozlabs.org>
Tested-by: NEddie James <eajames@linux.vnet.ibm.com>
Tested-by: NJoel Stanley <joel@jms.id.au>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>