clocksource/drivers/mxs: Convert init function to return error

The init function does not return any error, it prints a message, returns and
lets the caller unaware if the state of the system.

Change that by converting the init functions to return an error conforming
to the CLOCKSOURCE_OF_RET prototype.

Proper error handling (rollback, errno value) will be changed later case
by case, thus this change just return back an error or success in the init
function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>