dt-bindings: serial: Add common rs485 binding for RTS polarity

dt-bindings: serial: Add common rs485 binding for RTS polarity

rs485 allows for robust half-duplex serial communication.  It is often
implemented by attaching an rs485 transceiver to a UART.  The UART's
RTS line is wired to the transceiver's Transmit Enable pin and
determines whether the transceiver is sending or receiving.

Examples for such transceivers are Maxim MAX13451E and TI SN65HVD1781A:
https://datasheets.maximintegrated.com/en/ds/MAX13450E-MAX13451E.pdf
http://www.ti.com/lit/ds/symlink/sn65hvd1781a-q1.pdf

In the devicetree, the transceiver itself is not represented, only the
UART is.  A few rs485-specific dt-bindings already exist and these go
into the UART's device node.

This commit adds a binding to set the RTS polarity.  Most (if not all)
transceivers require the Transmit Enable pin be driven high for sending,
but in some cases boards may negate the pin and RTS must then be driven
low.  Consequently the polarity defaults to active high but can be
inverted with the newly added "rs485-rts-active-low" binding.

Document this binding in rs485.txt and in the two drivers fsl-imx-uart
and fsl-lpuart that are about to be amended with support for it.

Curiously, the omap_serial driver defaults to active low and already
supports an "rs485-rts-active-high" binding to invert the polarity.
This is left unchanged to retain compatibility, but the binding is
herewith documented.

Cc: Mark Jackson <mpfj@newflow.co.uk>
Cc: Michał Oleszczyk <oleszczyk.m@gmail.com>
Cc: Rafael Gago Castano <rgc@hms.se>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>