ARM: dts: imx6ul: add ARM architected timer

Add per-core ARM architected timer. Unfortunately bootloaders (U-Boot)
currently do not make the necessary initialization. Also specifing the
clock manually using the clock-frequency property seems not to help.
Therefor leave the timer disabled by default for now.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>