dt-bindings: serial: imx: clarify rs485 support usage

The i.MX UART peripheral uses the RST_B signal as input, and CTS_B as
output. This is just like the DCE role in RS-232. This is true
regardless of the "DTE mode" setting of this peripheral.

As a result, rs485 support hardware must use the CTS_B signal to control
the RS-485 transceiver. This is in contrast to generic rs485 kernel
code, documentation, and DT property names that consistently refer to
the RTS as transceiver control signal.

Add a note in the DT binding document about that, to reduce the
confusion somewhat.

Signed-off-by: Baruch Siach <[e-mail]>
Acked-by: Uwe Kleine-König <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/fsl-imx-uart.txt

···
 - fsl,dte-mode : Indicate the uart works in DTE mode. The uart works
                   in DCE mode by default.
 - rs485-rts-delay, rs485-rts-active-low, rs485-rx-during-tx,
-  linux,rs485-enabled-at-boot-time: see rs485.txt
+  linux,rs485-enabled-at-boot-time: see rs485.txt. Note that for RS485
+  you must enable either the "uart-has-rtscts" or the "rts-gpios"
+  properties. In case you use "uart-has-rtscts" the signal that controls
+  the transceiver is actually CTS_B, not RTS_B. CTS_B is always output,
+  and RTS_B is input, regardless of dte-mode.
 
 Please check Documentation/devicetree/bindings/serial/serial.txt
 for the complete list of generic properties.