clocksource: tcb_clksrc: Make tc_mode interrupt safe

tc_mode() can be called from interrupt context and thus must not call
clk_*prepare*() functions.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>