ARM: ixp4xx: Switch to use new timer driver

This augments the IXP4xx to select and use the new
timer driver in drivers/clocksource and removes the old
code in the machine.

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