char/cadence_uart: Remove TX timer & add TX FIFO state

char/cadence_uart: Remove TX timer & add TX FIFO state

This tx timer implementation is flawed. Despite the controller
attempting to time the guest visable assertion of the TX-empty status
bit (and corresponding interrupt) the controller is still transmitting
characters instantaneously. There is also no sense of multiple character
delay.

The only side effect of this timer is assertion of tx-empty status. So
just remove the timer completely and hold tx-empty as permanently
asserted (its reset status). This matches the actual behaviour of
instantaneous transmission.

While we are VMSD version bumping, add the tx_fifo as device state to
prepare for upcomming TxFIFO flow control. Implement the interrupt
generation logic for the TxFIFO occupancy.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 7a208a7eb8d79d6429fe28b1396c3104371807b2.1388626249.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>