arm: dts: mt7623: add clock-frequency to the a7 timer node to mt7623.dtsi

We need to tell the driver what the timers frequency is and that the core
has not be configured by the bootrom. Not doing so makes the unit not
boot.

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>