ARM: ep93xx: switch clockevent to timer 3

If we switch clock events to timer 3 we will have more bits to
use and can sleep longer when using NO_HZ.

Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>