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

Update the description of the binding.

- Describe the switches, which SoCs they are in, or if they are standalone.
- Explain the various ways of configuring MT7530's port 5.
- Remove phy-mode = "rgmii-txid" from description. Same code path is
followed for delayed rgmii and rgmii phy-mode on mtk_eth_soc.c.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>