pl011: preserve RTS and DTR signal on UART init

pl011: preserve RTS and DTR signal on UART init

Although we do not support hardware flow control in the Xen driver
for the PL011 UART, the other end may be configured to use it.
In this case it waits in vain for the RTS signal to be asserted by
the host and will never transmit any characters.
So we leave RTS and DTR as they had been setup before.
This fixes the UART input on Calxeda Midway, which uses hardware
flow control for the serial-over-LAN functionality.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>