dt_bindings: rs485: Add receiver enable polarity

dt_bindings: rs485: Add receiver enable polarity

Add polarity property for receiver enable. Some UARTs have separate
enable pins for tx (RTS or DE) and rx (RE). As most RS485 transceivers
have !RE pin, the default is active low in contrast to
rs485-rts-active-low that defaults to active high.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220426122448.38997-4-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>