dt-bindings: timer: cadence_ttc: Add power-domains

Describe optional power-domain property to fix dts_check warnings.
The similar change was done by commit 8c0aa567 ("dt-bindings: gpio:
fsl-imx-gpio: Add power-domains").

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/cc655a72b20790f6d7408b1aaf81c4bf878aafb4.1634286552.git.michal.simek@xilinx.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>