clocksource/drivers/digicolor: Fix warning of non-static function

Change the dc_timer function to be static as it is not used outside
this driver. This fixes the following warning:

drivers/clocksource/timer-digicolor.c:66:24: warning: symbol 'dc_timer' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Baruch Siach <baruch@tkos.co.il>