arm: dts: mt7623: add clock controller device nodes

Add clock controller nodes for MT7623, including topckgen, infracfg,
pericfg and apmixedsys. This patch also cleans up two oscillators that
provide clocks for MT7623. Switch the uart clocks to the real ones while
at it.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>