tty: xuartps: Add support for setting modem control signals

Add support for setting the state of the DTR and RTS signals.

Acked-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>