dt-bindings: net: dsa: mediatek,mt7530: improve binding description

Fix inaccurate information about PHY muxing, and merge standalone and
multi-chip module MT7530 configuration methods.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230212131258.47551-1-arinc.unal@arinc9.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>