openrisc/time: Fix symbol scope warnings

Spare reported the following warnings:
    arch/openrisc/kernel/time.c:64:1: warning: symbol 'clockevent_openrisc_timer' was not declared. Should it be static?
    arch/openrisc/kernel/time.c:66:6: warning: symbol 'openrisc_clockevent_init' was not declared. Should it be static?

This patch fixes by:

 - Add static declaration to clockevent_openrisc_timer as it's used only in
   this file.
 - Add include for asm/time.h for openrisc_clockevent_init declaration.

Signed-off-by: Stafford Horne <shorne@gmail.com>