clocksource/drivers/clksrc-probe: Introduce init functions with return code

Currently, the clksrc-probe is not able to handle any error from the init
functions. There are different issues with the current code:
 - the code is duplicated in the init functions by writing error
 - every driver tends to panic in its own init function
 - counting the number of clocksources is not reliable

This patch adds another table to store the functions returning an error.
The table is temporary while we convert all the drivers to return an error
and will disappear.

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