mach-u300: cleanup clockevent code

Use the new clockevents_config_and_register() function to register
the U300 clockevent, since that code requires ->cpumask to be set
we set this even on this UP system to please the framework.

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