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.