Documentation: dt: kona-timer: Add clocks property

The frequency for the Kona timer can either be specified through the
device tree or determined by checking the rate of the clock specified
in the device tree.  Update the documentation to reflect both ways.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>