devicetree: cadence_ttc: Document binding for timer width

Modern TTC implementations can extend the timer width to 32 bit. This
feature is not self identifying so the driver needs to be made aware
via device tree.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>