mck is needed to get the PIT working. Explicitly prepare_enable it instead
of assuming it is enabled.

This solves an issue where the system is freezing when the ETM/ETB drivers
are enabled.
Reported-by: NOlivier Schonken <olivier.schonken@gmail.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>