dt-bindings: serial: Add binding for rs485 bus termination GPIO

Commit e8759ad1 ("serial: uapi: Add support for bus termination")
introduced the ability to enable rs485 bus termination from user space.
So far the feature is only used by a single driver, 8250_exar.c, using a
hardcoded GPIO pin specific to Siemens IOT2040 products.

Provide for a more generic solution by allowing specification of an
rs485 bus termination GPIO pin in the device tree.  An upcoming commit
implements support for this pin for any 8250 driver.  The binding is
used in device trees of the "Revolution Pi" PLCs offered by KUNBUS.

[Heiko Stuebner converted the binding to YAML, hence his Signed-off-by.]

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/4f4411895dc52c846f766e7f7207ec412c4811b6.1589811297.git.lukas@wunner.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>