dt-bindings: serial: 8250_omap: drop rs485 properties

The binding references rs485 schema, so there is no need to list its
properties.  Simplify a bit by removing unneeded entries.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230124091916.45054-4-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>