arm64: dts: mt7622: add clock controller device nodes

arm64: dts: mt7622: add clock controller device nodes

Add clock controller nodes for MT7622 and include header for topckgen,
infracfg, pericfg, apmixedsys, ethsys, sgmiisys, pciesys and ssusbsys
for those devices nodes to be added afterwards.

In addition, provides an oscillator node for the source of PLLs and dummy
clock for PWARP to complement missing support of clock gate for the
wrapper circuit in the driver.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>