clocksource/drivers/armada-370-xp: Make syscore_ops static

The driver does not export armada_370_xp_timer_syscore_ops so
make it static to fix the following warning:

drivers/clocksource/time-armada-370-xp.c:249:20: warning: symbol 'armada_370_xp_timer_syscore_ops' 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: Gregory CLEMENT <gregory.clement@free-electrons.com>