timer: add OF_PLATDATA support for timer-uclass

The timer-uclass depends on full OF_CONTROL through its interrogation
of /chosen and the code to determine the clock-frequency.

For the OF_PLATDATA case, these code-paths are disabled and it becomes
the timer driver's responsibility to correctly set the clock-frequency
in the uclass priv-data.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>