Some boards provide GPIO controllable termination resistor. Provide
binding to make use of it.

Link: https://lore.kernel.org/r/20210818071232.20585-2-o.rempel@pengutronix.deSigned-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>