clocksource/drivers/ixp4xx: Add OF initialization support

This adds support for setting up the IXP4xx timer driver from
device tree.

Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>