tty/serial: atmel_serial: Fix device tree documentation

RTS pin is an active low pin.
For now, this doesn't change anything as the ACTIVE_LOW flag is not
handled in atmel_serial, but it will be in 3.16.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>