dt-bindings: net: dp83826: support TX data voltage tuning

Add properties ti,cfg-dac-minus-one-bp/ti,cfg-dac-plus-one-bp
to support voltage tuning of logical levels -1/+1 of the MLT-3
encoded TX data.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Catalin Popescu <catalin.popescu@leica-geosystems.com>
Signed-off-by: David S. Miller <davem@davemloft.net>