dt-bindings: arm: mediatek: sgmiisys: Convert to DT schema

Convert mediatek,sgmiiisys bindings to DT schema format.
Add maintainer Matthias Brugger, no maintainers were listed in the
original documentation.
As this node is also referenced by the Ethernet controller and used
as SGMII PCS add this fact to the description.
Move the file to Documentation/devicetree/bindings/net/pcs/ which seems
more appropriate given that the great majority of registers are related
to SGMII PCS functionality and only one register represents clock bits.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>