clocksource/drivers/tcb_clksrc: Use tcb as sched_clock

Now that the driver is registered early enough, use the TCB as the
sched_clock which is much more accurate than the jiffies implementation.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>