clocksource/drivers/sp804: Implement delay timer

Some ARM machines such as the Versatile and Integrator/CP
does not have any delay timer at all and have to rely on
the loop calibration.

We can use the SP804 clocksource for delay timers so let's
do it for those machines specifically.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>