ARM: ep93xx: move timer to its own file

This breaks the timer code out of the core file in preparation
for refactoring.

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>