dt-bindings: serial: document rs485 bindings for various devices

Atmel USART, Freescale UARTs and OMAP UART all support the rs485 binding
described in rs485.txt, this commit just makes that explicit.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>