clocksource/drivers/tcb_clksrc: Save timer context on suspend/resume

On sama5d2, power to the core may be cut while entering suspend mode. It is
necessary to save and restore the TCB registers.

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