serial_pl011: Set RTS during initialization

If the pl011 is connected to another device which has hardware
flow-control on, characters are never received by the pl011.
Asserting RTS when flow-control is off will have no effect.

This is in line with how Linux behaves.

Signed-off-by: Joshua Housh <joshua.housh@calxeda.com>
Tested-by: Marek Vasut <marex@denx.de>