x86: time_XX.c declare functions before they get used

Declare time_init() in asm-x86/time.h

Also did cleanup in asm-x86/timer.h :
timer_ack is only required for X86_32
int recalibrate_cpu_khz(void) is for X86_32

Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>