clk: pxa: pxa3xx: add missing os timer clock

The pxa3xx scheduler relies on the pxa-timer, which requires a clock for
its rate. As the clock handling will be taken over by the clock
framework, add this missing clock.

The miss was discovered by attempting to run a zylonite platform in a
device-tree configuration, with the future patch to shift clocks
handling to clock framework applied.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Michael Turquette <mturquette@linaro.org>