dt-bindings: clock: Add compatibles for MT7981

Add compatible string for MT7981 to existing bindings at
 - mediatek,apmixedsys.yaml
 - mediatek,topckgen.yaml
 - mediatek,ethsys.txt
 - mediatek,infracfg.yaml
 - mediatek,sgmiisys.txt

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/cc85ee470c781ff4013f6c21c92c0a21574b12b2.1674703830.git.daniel@makrotopia.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>