clocksource: time-armada-370-xp: Fix sparse warning

drivers/clocksource/time-armada-370-xp.c:217:13: warning: symbol
'armada_370_xp_timer_init' was not declared. Should it be static?

Also remove the __init marking in the prototype as it's
unnecessary and drop the init.h file.

Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>