dt-bindings: net: adin: document phy clock output properties

The ADIN1300 supports generating certain clocks on its GP_CLK pin, as
well as providing the reference clock on CLK25_REF.

Add DT properties to configure both pins.

Technically the phy also supports a recovered 125MHz clock for
synchronous ethernet. However SyncE should be configured dynamically at
runtime, so it is explicitly omitted in this binding.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>