i386: move PIT function declarations and constants to correct header file

setup_pit_timer is declared in asm-i386/timer.h.  Move it to the pit header
file, so it can be used by x86_64 as well.

Move also the PIT constants.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>