clocksource: cadence_ttc: Reuse clocksource as sched_clock

Reuse the TTC clocksource timer as sched clock provider.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Michal Simek <monstr@monstr.eu>