clocksource: add common i8253 PIT clocksource

This is based upon both arch/arm/mach-footbridge/isa-timer.c and
arch/x86/kernel/i8253.c.

Acked-by: John Stultz <john.stultz@linaro.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>