dt-bindings: arm: mediatek: document clk bindings for MT7629

This patch adds the binding documentation for apmixedsys, infracfg,
pciesys, pericfg, topckgen, ethsys, sgmiisys and ssusbsys for MT7629.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>