arm: dts: mt7623: add clock-frequency to CPU nodes

Add clock-frequency property to CPU nodes. Avoids warnings like
[    0.001568] /cpus/cpu@0 missing clock-frequency property
[    0.001588] /cpus/cpu@1 missing clock-frequency property
[    0.001601] /cpus/cpu@2 missing clock-frequency property
[    0.001614] /cpus/cpu@3 missing clock-frequency property
at boot time

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