dt-bindings: net: dsa: mediatek,mt7530: define phy-mode per switch

Define acceptable phy-mode values for the CPU ports of mt7530 and mt7531
switches. Remove relevant information from the description of the binding.

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