clocksource: cadence_ttc: Add support for 32bit mode

New TTCs support 32bit mode. Older versions support
only 16bit modes. Keep 16bit mode as default
and 32bit optional.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>