dt-bindings: mediatek: update bindings for MT7629 SoC

This updates bindings for MT7629 SoC, which includes very basic items
such as system timer, UART, sysirq and scpsys unit.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>