clocksource: convert footbridge to generic i8253 clocksource

Convert the footbridge isa-timer code to use generic i8253 clocksource.

Acked-by: John Stultz <john.stultz@linaro.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>