dt-bindings: clock: mediatek: document clk bindings for MediaTek MT7622 SoC

This patch adds the binding documentation for apmixedsys, ethsys, hifsys,
infracfg, pericfg, topckgen and audsys for MT7622.

Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>