clocksource/drivers: Add an alias macro CLOCKSOURCE_OF_DECLARE

The macro CLOCKSOURCE_OF_DECLARE has been rename to TIMER_OF_DECLARE.

In order to prevent conflicts for the next merge window, a temporary
alias has been added which will be removed later.

Cc: Arnd Bergman <arnd@arndb.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>